IIRC Win7 was a little quirky with UEFI. It does support it; big OEMs like Dell and HP installed Win7 on UEFI-based systems from 2012 onward. However with home built systems, I've always had better luck installing it in legacy mode.

Win10 is easy-peasy. You can switch back and forth between UEFI and legacy mode as often as you'd like and it will continue to work as if nothing happened. Since you have XP in the mix, I'd recommend just using legacy mode for all operating systems.

When it comes to storage controller modes, Win XP and 7 would very much appreciate it if you'd just pick one during installation and stick with it, thank you very much. Win10 is far more lenient and won't mind if you toggle between AHCI and IDE. Since you have an SSD, I'd recommend using AHCI for all operating systems. SSD controllers were built with AHCI features in mind, so you'd lose performance in IDE mode.

I use AHCI on all my XP rigs just fine both Intel and Nvidia chipsets.
Not use about UEFI, Personally I prefer the simplicity of of legacy BIOS but that's just personal choice over any technical reason.

Thank you guys! I used the Legacy and AHCI combination and now my operating systems are well installed and clean.

But I'm not sure if I broke anything. Previously, games on Windows 7 (Diablo II 1.14d version and Death Rally, two examples I just tested) started and exited quickly and without any problems. Now, not only do they take a long time to open and exit (most of the time staying "not responding", but the installers themselves were all stuck and laggy. Battle.net was like that too, although I found a solution where I disabled an automatic configuration in the LAN properties and it came back to normal.

Is it possible that having it installed on AHCI caused this, or is there some other possible reason?

Killermac wrote on 2024-01-07, 23:55:

Is it possible that having it installed on AHCI caused this, or is there some other possible reason?

I apparently found the problem and wanted to share, although I feel a little embarrassed and it's not related to the original question. I thought I had installed the sound card driver, but I didn't and I connected my headset to its output. I suspected there was something related to the sound because when shutting down, Windows would freeze the "logoff sound" process as well.

Welp, I think that's it. Living and learning. Thanks again chinny and Steve!

Standard Def Steve wrote on 2024-01-06, 21:49:

IIRC Win7 was a little quirky with UEFI.
[…]
I've always had better luck installing it in legacy mode.

AFAIK, Vista/7 (64 bit only) support EFI only as far as having an EFI port of bootmgr, but don't support most other innovations of it - you still need an [emulated] AT keyboard, VESA or at least VGA compatible graphics (as opposed to EFI's GOP), possibly gate A20 and whatever "innovations" in memory mapping came later, etc…

Depends on your needs, but switching between CSM and native (U)EFI is a pain though.
Personally I think it's better in the long run to keep everything Legacy/CSM in multi old OS systems (especially if you don't want to and/or can't install modified versions of WinXP/Win7 with relevant patches applied).
Some MBs have firmware capable of (U)EFI boot on CSM enabled settings (it simply shows ALL options to boot from, regardless of type), if you ever want to EFI boot Win10 and Legacy older ones without BIOS trip.

I would do AHCI whenever possible (moded drivers and all), since it will help performance.

I use both UEFI and CSM booting on my Devil's Canyon rig (Intel H97 chipset). I switch SSDs for each OS using an external SATA enclosure, as well as a NVME drive internally for Windows 7 64-bit (set to boot the Windows Boot Manger first) . I use the boot selection menu to boot in legacy mode off of a SSD, optical drive, or USB as needed. No issues.

As for AHCI vs SATA, I recommend AHCI on Intel chipsets. I've had weird speed issues on my 60 and 70-series motherboards when using SATA mode. AHCI mode resolved the issue on these boards. I don't understand why it happens.

IDE just should be a matter when booting legacy operating systems.
For XP and above, AHCI should be fine - and faster.

FYI, Windows 2000 can use AHCI drivers too. Search for Fernando's AHCI driver pack. Works great!

Rlowe's AHCI drivers for Win9x also work wonders.

There are also AHCI drivers for OS/2 (3.0 and later), but you have to pay for a license. I grabbed the one on the Ecomstation live CD for free.