First, try digging into the user manual that came with your motherboard.
Look for the section that tells you which Hardware Interupts (IRQs) are assigned to which PCI slots, and also which on-board peripherals share the IRQs assigned to these slots.
My guess is that you are trying to put the X-Fi into the PCI slot that shares the IRQ assigned to the AGP or PCI-E slot intended for your Video card or for an on-board video chipset.
Just because the slot was empty doesn't mean that you can put a card there without causing a conflict with some other piece of pre-existing hardware.
Some cards/peripherals can share IRQs and slots. Others cannot. Usually USB, Ethernet, and Modem cards will share IRQs, but Sound and Video cards often will not.
The object is to find the correct PCI slot to plug your X-Fi into.
Search your manual and try to find which PCI slot shares the IRQ for the on-board sound chipset (usually IRQ 5), and after disabling the on-board sound in the bios settings, installing the new X-Fi card into that slot.
If some other card is already installed there, move that card into another open PCI slot. Perhaps it is capable of sharing IRQs with some other device.
Sometimes it just comes down to trying to find the right combination of cards and peripherals that can best share the available IRQs and slots. Just keep trying to swap out one card for another until it works, or as a last resort you can make a decision about which card must go to make room for the new one.
Good Luck.
CptA...Well since you got it fixed...Never Mind.