It's definitely a good idea to make a Ghost copy of the drive first, so if whatever you try messes it up, you can always re-ghost the drive and try again.
I don't have a whole lot of time right now but I'll check back tomorrow and if you still haven't fixed it, I may have more info for you.
Knoppix (as mentioned above) should at least let you get to the data.
There's a bootdisk method that allows you to change the Administrator password to anything you want. A commercial version is here:
http://www.lostpassword.com/windows-xp-2000-nt.htm
and it may work fine, but I don't know if it's worth $200..
I've used a free version to recover a Win2k password -- it's a Linux bootdisk and works just about as well, but it's been a while and I can't remember the name or source.
This might be the free version of the boot disk:
http://www.pchelplive.com/article.php?sid=70
I don't think @stake's LC4 will work if you retrieve the SAM file using ntfsdos or linux, as the samfile's probably protected with SYSKEY. But you still may be able to reset that password with the bootdisk-method.
Can you login to the box as any other user at all? It doesn't sound like it but if you can, I can explain a way to find out what the current Admin password is using the AT scheduler and the bootdisk-method. Let me know if you're interested....
Sorry, I gotta get going but I'll check back tomorrow and hopefully will have more info if you haven't already fixed it by then. Good luck!
p.s. I think if you reinstall Windows, that necessarily will erase the My Documents folder. Also, I know WinXP does hardware checking, so you if your mobo is dead you might need the same model mobo to replace it, and even then it may still not work...I know you can't move the same install of WinXP between different hard disks as it does hardware-specific checking.
Good luck!