Try using a PS/2 mouse and see if it makes any difference. Also remove any other USB devices you might have plugged in.
The other option would be to try and get the BIOS to assign a different IRQ or reserve a different IRQ for the PCI card. Every BIOS i've seen before allows you to assign a specific IRQ to a specific PCI slot, it's generally in PCI device configuration or something to that affect.
You could also try putting the card in a different PCI slot, this tends to assign a different IRQ (it also makes windoze 98 re-install the driver
).