If you just want to get the data off, pull the hard disk drive out and plug it into another Windows machine.
If you actually want to logon and use the machine again, reset the Administrator password (which doesn't require pulling out the hard drive, and may end up being easier). As long as he wasn't using encrypting filesystem (EFS), just download the ISO image for this password reset application and reset the Administrator password: http://home.eunet.no/pnordahl/ntpasswd/bootdisk.html
I've used this particular password reset application for years and only have good things to say about it. As the web site for the application says, though, if you were using encrypting filesystem then changing the password this way will render the encrypted data lost forever. BTW, if it's Windows XP Home Edition then you need not worry about encrypted data-- EFS doesn't work on XP Home.
Have you booted in "Safe Mode" yet? If not, try that. If "Safe Mode" doesn't work, describe what you see when you try to boot in "Safe Mode".
It sounds like you might have either (a) a hardware problem, or (b) malicious software that has screwed up Windows. (It could be that USERINIT.EXE is screwed-up. That's what runs immediately after you logon, and I've heard anecdotal reports of malicious software rendering the machine into a state you describe after toying with USERINIT.EXE.)
"Safe Mode" will probably get you in. If you do get in, copy the data off to a USB memory stick, etc, and plan on leveling and re-building the machine.