You can't do that. You will need to back up all of your critical data and then do a fresh install of windows with all of the new hardware. Windows will give you BSOD if you try to use an existing install of any XP OS.
Edit: you can use your XP cd and Kay again. If you get a mesage saying that it has ben used to many times you just do a telephone activation as many times as you neeed. You just tell the MS outsourced chick in India that you have done a major hardware upgrade and need to manually acitvate your copy of XP!!
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.