Yes, Mbox works like a charm with XP.
Be warned, however, that if your band's recording have more than 32 tracks, you will only be able to play back the first 32 with the Mbox (or 002, 001,
002r) versions of PT. This may be "fixed", to a degree, with the next version of PT (due out any day, now) which will let you sellect which tracks (up to 32) you want to hear at any given point in the song.
I own a 002, but used an Mbox at a clinic digi is running these days- its a pretty sweet little unit.
If you are planning to conitinue working with studio's using protools, it might be worth your while to return the Quattro (which will only work with Pro Tools FREE, and not on XP) and buy an Mbox. And tell the guy at Guitar Center to do his @#$!ing homework.
Take care,
Chris