You could use Knoppix, but the thing is, how do you know what to repair.

Have you tried this:

you might try ctrl/alt/del twice at the login screen and enter Administrator with no password... it may allow you to get into the machine.

You could also use one of those prebuilt XP tool cds. I don't know where you'd get anything like that, but do a search and I am sure you can find it. They allow you to boot Xp, and repair damaged files and so on.