you are going to want to run the checkdisk utility:
enter the command prompt from boot then type chkdsk /f and you should be able to find the error.
otherwise you can boot from your XP CD and diagnose from there.
You can try a repair install of windows xp by booting to the xp disk and see if it will repair it first. If you try the repair and it tells you it cannot find an instance of xp to repair then you are facing a format/reinsstall of the OS.