move the cards around.

Also win98 doesn't like usb 2.0, so avoid it.

I've tried all the PCI slots and changing the VGA card too. Two of the expansion cards I've tested are 1.1, the other (2.0, Belkin) says that i's Windows 98 compatible.

well, you got two cards fighting over resources. Try allocating irq/dma/memory ranges manually.

VX Pro... A dreaded PCChips motherboard. What kind of chipset does USB 2.0 card have? Is it VIA, NEC or OPTi?

Try to clean the contacts of all the cards (including the USB 2.0 one) and the RAM modules, you can use an abrasive eraser to make them squeaky clean, oxidation can cause a lot of weird things. If you have access to another PSU, try it. It's not normal that a BIOS resets itself every time you put a USB 2.0 card inside. Consider replacing the CMOS battery if not now and if possible, do a BIOS update. If it keeps having issues, you might have a defective motherboard, and since it's PCChips, it wouldn't be surprising.

luckybob wrote:

Also win98 doesn't like usb 2.0, so avoid it.

I think 98SE is fine with USB2.0, after you install NUSB.

I noticed that in the system info screen that appears after POST, that VGA has no IRQ assigned. There's an "Assign IRQ for VGA" in BIOS disabled by default, but activating it doesn't seem to change anything.

Windows 98 hangs with every USB card I've tried, it does not matter if it's 1.1 or 2.0, but DOS loads fine (because I suppose that doesn't recognize any).
If I wait long enough after the hang, I get a BSOD that indicates that it's not possible to write in C:. If I select the DMA option for the hard drive in Device Manager I get similar results.

I have already upgraded the BIOS and I have tried a W98 reinstall. The RAM has been installed in the board for more than 20 years and the contacts in the cards all seem clean to me. I can't try another PSU, but taking apart the SB16 ISA doesn't seems to do anything. I could try disconnecting the second hard drive.

Could be something related with IDE controller or it's just a defective motherboard?

While they might seem clean, that doesn't mean they are. Try to clean the contacts, you got nothing to lose after all. Too bad you don't have another PSU, a spare one is always good to have around just to rule out possible power issues. If you have one, try a PCI ATA133 controller, the Silicon Image SIL0680 is a pretty good one and has Windows 98 drivers, just make sure you set the boot device to SCSI (although on some motherboards it's detected as a bootable IDE controller). How old is the hard drive? It might not be just the motherboard that is defective.

Did you test the RAM with Memtest86+?