You can use the repair option with the XP disc, or use system restore either in Windows or with the disc (press F2)
Or do an overlay install to the same HD and folder. (never tried this myself though)
I would just do what AND_YOU_ARE said. Copy all important files to another HD or partition or CD and format.
TheMajorMMX - Intel P200 MMX @ 225Mhz - V-Tech (PcPartner) Baby-AT mb w/ Intel Triton TX chipset - Nvidia Riva TNT2 M64 w/ 16MB - 256MB M-Tec SDRAM - Still running.....