Tyler is right....you simply put the XP cd in with your desktop active and you will be given the option, in a drop down, to do an upgrade or full install...chose upgrade and your old system will be backed-up and a restore point created in case the XP install fubares!
Noli Nothis Permittere Te Terere!!
EVGA 680i, Dual EVGA 8800GT'S (650/950), 4 gigs OCZ DDR2 1066, Intel Q9450 at 3.8 Ghz, Koolance Water Cooled, Dual Plextor16x DL DVD+/-RW. Dual WD 250gb 16mb Cache Sata 3.0. 3DMark06 score = 19,168.