With WinXP you can run up to a 120gig PART. and with SP1 you can run as large as you want (as far as I know)
I have a 160gig and had to Part. it out at 100 and 60 to get windows and my ststem to work right. After I put SP1 on it then it would work fine at full 160gig.
So if in doubt, run a 120gig or smaller for your boot drive and what ever you want as a back up