OK I am not an expert, but have you used the disk checker?
You might have a corrupted HDD.
My Computer -> Right click C: (if that's the one with XP One)-> Properties -> Tools Tab -> Error checking, click the Check Now button.
I would put a tick in BOTH boxes ("automatically fix file system errors" and "scan for and attempt recovery of bad sectors") Then click the check now button.
I have a funny feeling that this process can't start until you restart the PC, but if that's the case, you get a pop-up to tell you that.
If so, just restart.