First off: welcome! And nice machine you got there 😀
Now, as for those questions:
I want to downgrade to windows 98, but I’m not sure what the best method in doing so is. I don’t have any software disks. I’m not sure where to get the correct drivers either.
Good news: this machine predates Windows XP, in fact it would have been shipped with Windows 98SE. So there definitely are drivers for all components.
Windows 98SE needs installation media and a valid product key. Microsoft no longer sells it, but you might find original disks. You needn't worry about keys no longer being valid, there was no online activation in 98 days to prevent re-use.
For drivers, get them all together before installation. Back in the day I burnt them onto a CD. These days I'd use either a CF-card with CF-to-IDE adapter, or a spare hard disk formated to FAT16 or 32 (DOS/Win98 file systems). Hook that up alongside the main hard disk while installing and you're good to go.
Of course you need to know which ones you need. For that, look at the chips. You don't mention the most vital component of the system: the motherboard. If it's an i440BX-based board (check the big square chips - if they say "Intel" you're in luck), drivers are included in Win98SE installation. If it's a Via chipset board, you need separate Via 4-in-1 drivers, and it's important to install these before other drivers such as video card and sound card. After that, do video and sound. The nVidia card needs nVidia Detonator drivers. The sound card is a Sound Blaster Live Value, which can be a real pain, as there were many different versions and many different drivers which only installed on specific cards. If you can find something for that card and have the choice, take .vxd drivers. not .wdm.
Apart from that none of the devices you mention need drivers. You might have a network adapter though, it will probably need drivers.
First stop for drivers is www.vogonsdrivers.com. You should find everything I referred to there.
The hdd is making a constant whining noise and the pc is extremely slow, sometimes not even responding when trying to open files and programs. I’m guessing the hdd needs to be replaced. I’d like to replace it with a small ssd if possible. But I’m not sure how to do that.
That sounds (literally) like a dying HDD. Replace ASAP.
SSD sounds simple but can be rather tricky - even small SSDs might be too large for motherboard BIOS, SSDs are SATA, so need either a SATA-PATA adapter or a PCI SATA card that works in your old motherboard. If you manage to get the hardware sorted you then hit the fact that you need to align partitions to sectors on an SSD, and Windows 98/DOS FDISK can't do that so you need to partition and format the drive elsewhere under a different OS. I've done it on a similar system, using a vintage Intel X25E 64GB SSD and a Promise SATA150 TX2Plus controller, but it was a bit of a bumpy ride - not recommended for a first time.
Better idea: a compact flash card or a Disk-on-Module. CF actually uses the IDE interface, so all you need to get it to work is a passive CF-IDE adapter. Because it's passive it's pretty foolproof, unlike active SD-IDE or SATA-PATA adapters. A 16GB CF card would be very affordable (EUR 19 where I live for a Sandisk 50MB/s unit) and more than fast enough to max out the board's I/O controller. All you need is an adapter, which can be bought on AliExpress for EUR 3 or so including shipping, or slightly more locally if in a hurry. Disk-on-Module is simply a flash disk you stick onto an IDE connector. They are rather old, and prices vary widely, but 16GB is doable and sometimes affordable on a famous auction site. De facto this will work the same as a CF card, just more elegantly as no adapters needed.
Hope this gives you some pointers in the right direction. If you need more detailed help, don't hesitate to ask 😀
