if i were you i would try a differnt pci slot first. the motu or any Good audio interface should be in slot that gives it its own irq. if sharing the irq, then it would result in poor operation. in other words do not install it in either the top or bottom pci slots. use one of the middle slots.
that being said, a bit more info about your rig could yeild helpfull information
for instance. i have a Gigabyte mobo, with an xp3400 chip, one gig of Kingston hyperx ram 400 mhz, a wd raptor 36g, 10,000 rpm hard dirve for os and programs, and a wd 72g 7200 rpm for audio storage, a simple agp 4x 128 dual monitor video card, and the motu 324 pci card, connected to a motu 24i. all wrapped up in a 4 rack space aluminum case with an antec 450 psu.
im useing xp pro with the new xp driver from motu s site. To date I have had good luck, no problems fireing up with or without the 24i connected or not. If i were you i was verify the card works properly, by installing into a different puter to see if it is reconized, be aware you ll need to install the drivers to see it correctly in the device manager..if you know the integrity of the pci card is ok, have you downloaded the xp drivers from Motu's site? these are only a few possiblilites.
be aware that some motherboards just dont play nicely with higher end audio pcs. give a lil more info about your rig and you may get a better idea of possible problems,
hope this helps