Got a similar problem, I have test-installed XP once and installed the latest mouse drivers.
Mine is a Logitech Optical USB and I have made double extra sure that mouse acceleration is off, and on the desktop, it IS OFF. Even Quake3 plays okay without acceleration (though I think that game accesses the mouse differently from other games - not sure!?) but when I start any other game (like Unreal) I got mouse acceleration back.
Even checked registry, hacked in it what I thought might do the trick, with regards of the driver documentation from Logitech (they explain every registry setting in a text file), but to no avail!!
As of that, XP is no choice for playing right now
