Always, always install the old OS first. Microsoft boot loaders and installers have a general tendency of not recognizing and thus overwriting any newer stuff they encounter...

I dont know what tools you have to do this but I presume you have the disc based media for Windows XP and Windows 98, a Windows 98 floppy disk and a fully working computer you want to install onto.

First you need to set up a partition for Windows 98 to install onto. As its a 250Gb disk you will need a bit of trickery to do it as Windows 98 wont play ball.
This part would best be done in another PC. You need to add a partition for Windows 98.
I would add the 250Gb disk to a PC and use that to put a partition on it to what ever size you want to use, 40Gb, 80Gb upto 127Gb for Windows 98. Provided your hardware can see a single 127Gb partition.
If you dont have another computer to do this you can use your Windows XP CD.
Boot using that, it will load drivers and then ask you how you would like to deal with your hard drive. At that point you tell it to create a partition of the size you want for Windows 98.
You can use the Windows 98 floppy disk for this, but it looks like it doesnt work properly all the time on all computers, and Ive had some computers where it wont work at all, which is why I tell you to do it like this using the Windows XP CD.
Once the parition is created turn off the PC and boot using the Windows 98 boot floppy disk.
There will be a partition waiting for you, so you can use format.com on the floppy disk to format it FAT32 and copy all your setup files from the Win98 folder to your hard disk and install Windows 98.
Once installed you will have Windows 98 occupying a small portion of the disk and some unpartitioned space.
You just need to insert the Windows XP CD once Windows 98 has booted and install it to the unpartitioned space on the hard disk.
You use the Windows XP auto run menu that runs as an app in Windows 98 from there you dont want to replace Windows 98 so you dont want to install it to C:

Once you get there it will become clear what you need to do, in that you do want to install but not over 98.

And thats it really. Quite simple once you get going.
If you make a mistake installing XP and replace 98 with it just do it all over again.

It really is better and easier to do the first part in another PC using that OS to create the partition and GUIFormat.exe to create the format.
You use GUIFormat.exe because WIndows doesnt like to put a FAT32 partition on large hard disks even though its capable of taking it.

ElectroSoldier wrote on 2023-10-16, 15:23:

I dont know what tools you have to do this but I presume you have the disc based media for Windows XP and Windows 98, a Windows […]
Show full quote

I dont know what tools you have to do this but I presume you have the disc based media for Windows XP and Windows 98, a Windows 98 floppy disk and a fully working computer you want to install onto.

First you need to set up a partition for Windows 98 to install onto. As its a 250Gb disk you will need a bit of trickery to do it as Windows 98 wont play ball.
This part would best be done in another PC. You need to add a partition for Windows 98.
I would add the 250Gb disk to a PC and use that to put a partition on it to what ever size you want to use, 40Gb, 80Gb upto 127Gb for Windows 98. Provided your hardware can see a single 127Gb partition.
If you dont have another computer to do this you can use your Windows XP CD.
Boot using that, it will load drivers and then ask you how you would like to deal with your hard drive. At that point you tell it to create a partition of the size you want for Windows 98.
You can use the Windows 98 floppy disk for this, but it looks like it doesnt work properly all the time on all computers, and Ive had some computers where it wont work at all, which is why I tell you to do it like this using the Windows XP CD.
Once the parition is created turn off the PC and boot using the Windows 98 boot floppy disk.
There will be a partition waiting for you, so you can use format.com on the floppy disk to format it FAT32 and copy all your setup files from the Win98 folder to your hard disk and install Windows 98.
Once installed you will have Windows 98 occupying a small portion of the disk and some unpartitioned space.
You just need to insert the Windows XP CD once Windows 98 has booted and install it to the unpartitioned space on the hard disk.
You use the Windows XP auto run menu that runs as an app in Windows 98 from there you dont want to replace Windows 98 so you dont want to install it to C:

Once you get there it will become clear what you need to do, in that you do want to install but not over 98.

And thats it really. Quite simple once you get going.
If you make a mistake installing XP and replace 98 with it just do it all over again.

It really is better and easier to do the first part in another PC using that OS to create the partition and GUIFormat.exe to create the format.
You use GUIFormat.exe because WIndows doesnt like to put a FAT32 partition on large hard disks even though its capable of taking it.

Right what i needed to know, thank you so much ElectroMan for your fast and clear answer man 😀

ElectroSoldier wrote on 2023-10-16, 15:23:

If you make a mistake installing XP and replace 98 with it just do it all over again.

oh God believe me when i say last year i did on the P3 machine and that was so sad and annoying 🙁 i don't remember how i screwed up, but i did.

Yeah i can use another PC to try creat the partition, sounds more easy. Actually i have both Windows XP Home Edition (and Pro but i'll use Home) the ORIGINAL cd with legit key! Also Windows 98 SE (Italian version) is original cd 😀 i think somewhere i should also have the floppy boot, but i don't know if is needed as far i remember for Win95 was essential for boot the setup, while with 98 i always used booting into CD without any problem so far.

The PC will be an Asus A7S8X-MX Socket 462 with SiS 741GX Chipset, AMD Sempron 2600+ Thoroughbred 1.833Mhz, 256\512MB DDR Ram, ESS Allegro-1 for both DOS and 3D Sound and a GeForce FX 5500 AGP8x 256MB. (is this configuration good for DOS and 98\XP era? don't mind of ultra detail or Doom 3 and Half-Life 2, just anything from 2003 and below, but expecially DOS)

By far i used always GParted for split in two one HDD and create partitions, it is good or should i use the program you told me? Sorry for all answers, im getting rusty (im 32) and an help is always great 😀 yeah so the 127GB limit was right, despite i think somewhere there was a patch made by a late dude R.Loew that bypassed\fix that. Is it good to use these patches? 'Cause being a more modern 98 build (for the "vanilla" i'll use the Pentium 3 machine), so i thought could put something like recognize SATA HDD's, 127 barrier fix and NTFS readable, if all of these are possible.

Thanks again for your guide, when this week the mobo will arrive i'll read your answer again and use your wise heads up!

You dont need to use the OS and GUIFormat.exe to set up the partition and format on the disk, its just what I use because I have the OS and GUIFormat.exe is free to download.
As long as you end up with the disk that has the partition size you want and formatted FAT32 then use what ever tools you want to.

I dont use any patches for Windows 98 myself so I couldnt tell you to use any of them because I dont have the experience in using them.
That setup sounds quite nice for Win98. I imagine it would be powerful enough to do at least most things you could want to do with 98 as that hardware came towards the end of the life of 98.
Developers ditched 98 pretty quickly after the introduction of XP as it was already known WinME was the end of the road for Win9x operating systems.

You dont need the Win98 boot floppy, you can use the CD to boot from to get to the Windows 98 installation routine no problem, the boot floppy method always works thats all. But as your hardware is towards the final years of 98 there should be no problem at all just using the CD.

ElectroSoldier wrote on 2023-10-16, 17:40:

You dont need to use the OS and GUIFormat.exe to set up the partition and format on the disk, its just what I use because I have […]
Show full quote

You dont need to use the OS and GUIFormat.exe to set up the partition and format on the disk, its just what I use because I have the OS and GUIFormat.exe is free to download.
As long as you end up with the disk that has the partition size you want and formatted FAT32 then use what ever tools you want to.

I dont use any patches for Windows 98 myself so I couldnt tell you to use any of them because I dont have the experience in using them.
That setup sounds quite nice for Win98. I imagine it would be powerful enough to do at least most things you could want to do with 98 as that hardware came towards the end of the life of 98.
Developers ditched 98 pretty quickly after the introduction of XP as it was already known WinME was the end of the road for Win9x operating systems.

You dont need the Win98 boot floppy, you can use the CD to boot from to get to the Windows 98 installation routine no problem, the boot floppy method always works thats all. But as your hardware is towards the final years of 98 there should be no problem at all just using the CD.

Thank you really so much pal 😄 i will let you update in case i'll have trouble, but i hope not 😀 the mobo and gpu will come anyway during this late week, hoping that everything will work just fine. And yeah W98 had a glorious life, thinking it had extended support until 2006 if i don't get wrong, the year where it definetily died. Yet indeed some motherboards from that year still somehow had support for Win98, or at least some models were still able to have all drivers working, and there were lot of mods for it (Unofficial Service Pack 3, KernelEX, Unofficial PCI-Express drivers ecc.). Don't know why i am so obsessed with it, but i had really great times with it and still can't abandon it even nowday, mostly for compatibility of that era games and, of course, DOS games. I can't help myself with the fact that despite how emulators (DOSBox, above all) are pretty good, they are anyway not 100% perfect or close to real reproduction so that's why i am still very attached to it.

Thanks for this nostalgia trip and for your advice again 😄

Yeah I think there was support until 2006 but development in any meaningful way stopped in in '03/04 because most people saw XP and wanted to use it because it looked so much nicer than 98 did.
Games had 9x support but they were being developed with XP in mind, software for business went that way to because it was as easy to dev for XP as it was for 98, and as the months went by it became even easier to dev for XP with all the tool kits you could get than made it easier. And thats one of the reasons XP stuck around for so long, a lot of businesses had a lot invested in the platform and it worked well once set up.
But I do still have a Win98 PC myself complete with CRT for playing those old games that only feel right on 98.