As usual, I apologize for the long post (tl;dr at the bottom).

I recently resumed building/testing my retro rig:
Mobo: SY-7VCA (latest BIOS)
CPU: Intel Pentium III 1000EB
RAM: 1x 512MB PC133
Storage: Gigabyte 120GB SDD with SATA-IDE adapter
GPU (AGP): GeForce FX5200
Network (PCI): Intel PRO/100+
Sound (ISA): SB AWE64 (CT4500)
Sound (PCI): SB Audigy 2 ZS (SB0350)
USB 2.0 (PCI): NEC-based 5-port

I started testing OS installation recently.
Windows 98 SE installed ok, but after a couple of restarts started misbehaving, mainly locking up.
I formatted the drive and tried Windows 2000, then XP, both locking up during installation.
I ran Memtest86+ and identified the RAM was faulty (lots of errors), so I replaced that stick and ran Memtest again for a long while. No more errors.

I then restarted my OS installation (always as standalone, formatting the drive before each installation; no dual boot):
Windows 98 SE: installed without issues, as well as its drivers.
Windows 95 RTM And OSR2: same
Windows Me: same
Windows NT 4.0 Workstation: Installation went smoothly; however on first login, right after entering credentials, the desktop didn't load. Only the default "green" background with the hourglass cursor. Trying to move the cursor showed that the machine froze. No activity. I waited around an hour and nothing changed.
Rebooted, same issue.
Windows 2000 Pro SP4: BSOD in the file copying step of the text-based installation, PAGE_FAULT_IN_NONPAGED_AREA - cdfs.sys / ntfs.sys
Windows XP Pro SP3: BSOD in GUI-based installation during "Installing devices" step, IRQL_NOT_LESS_OR_EQUAL

I don't have another SSD or HDD at the moment to try, but I doubt the issue is with the storage.

I then decided to try removing expansion cards.
Due to the case I'm using, removing the ISA is a pain-in-the-@ss, so I left that as a last resort.
I started by removing all 3 PCI cards: USB 2.0, Ethernet and Sound (Audigy 2 ZS).
I tried installing NT 4, Win2k and WinXP: all 3 installed successfully and worked fine after boot/login.

Next, I decided to test each PCI card and PCI slot individually, each time installing NT 4.0, Win2k and XP (again, always standalone, no dual boot).
Turns out the issue happens whenever a card is in PCI slot #4.

tl;dr:
If PCI#4 is populated, NT-based OSes freeze up or BSOD; however 9x-based OSes work fine.
If PCI#4 is not populated, all OSes work fine.
[endof tl;dr]

Before I conclude that the PCI slot is defective (which shouldn't be the case, as it seems to "work fine" under 9x), I wanted to ask for advice.
Is there one ore more BIOS settings that could be causing this?
Could this be due to some kind of IRQ conflict or similar? I'm a noob when it comes to IRQs, so any advice on how to further investigate/analyze/troubleshoot, I'd appreciate it.

If it helps, here's the mobo manual for reference (incl pics):
https://www.manualslib.com/manual/481231/Soyo … vca.html#manual

The manual includes pictures of the BIOS settings as well, but they may be outdated, and some settings may be different in mine (haven't compared). If needed I can take pics of my current BIOS settings and post them for review.

For those wondering why I don't simply use one of the remaining 2 PCI slots: one of them is too close to the ISA, and doesn't match the case's back cuttings. The other one is too close to the AGP slot, and would block the GPU's fan (which I fear may affect both cards, as the GPU won't be able to properly remove hot air, and the PCI card would get hot constantly).