IRQs

PCI cards need something called IRQ (Interrupt Request Que) to operate, though some do not. A discussion of what IRQs are is unnecessary, but you may just be aware that a motherboard needs a specific number of IRQs (usually 16). Usually, most of these are already in use leaving 6 or so IRQs free for expansion cards.

Some PCI cards need an IRQ exclusively. If two such expansion cards start using the same IRQ, problems will arise. Some PCI slots share IRQs themselves. So, devices that need exclusive IRQs should not be used in them.

Windows 9x/Me need IRQs assigned in the BIOS, while Windows 2000 assigns IRQs dynamically.