I am going to go out on a limb and say that if you can spare the information that is stored on the PC, and you just want to get it up and running, format the drive.

I'd suggest getting the Hard Disk utilities disk that your drive manufacturer makes available. Maxtor, Seagate, WD etc... all make tools for diagnostics/formatting etc.

Once you have that, just format the hard disk. Once formatted then use the XP boot disks to install XP.