i doubt it'll be something to do with the usb port... possibly a conflict. have you tried the mouse in a different usb port? i know it sounds soft as they're the same port, but ive heard once before about certain things working funnily in different ports.
also have you got any other usb devices plugged in? using a usb hub or something?
each usb port has a max power consumption of 500mA available to all devices plugged into it (unless you have a mains powered hub which will bypass that limit).
if my memory serves me correctly, i think the mouse only uses 100-200mA??
also... have you got the latest intellimouse drivers installed? (if not, then i would try that) and have you got the WinXP powertoys installed? (if so, uninstall em, if not, dont install em)