Your best bet is probably going to be formatting the hard disk and re-installing from scratch. (if the drive isn't damaged).
We're having the same problem with a machine this week over at the lady-friend's house. Here's the link we came up with:
http://www.help2go.com/Tutorials/Win...r_corrupt.html
Though that page is primarily about what to do if your system file is missing, it's got a good deal of info about dealing with issues with XP when you can't get into the OS.
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