first of all we need to know witch of the ports your're using
the simple serial bus ore the USB ? So if you're using the USB connection unter os XP i thinkt you should have installed service pack 2 and the right driver - just try to get the newest driver for the system, perhaps this will solve the problem.
and if not: did it work before ? Where did you buy it ... maybe you might have a damaged unit, but i don't think so - sounds more like a driver/incompatiblity problem.