I've read about one way of doing it.. but I have no clue if it actually works, and I take no responsibility if something goes wrong.
1. Restart your comp.
2. Use F8 to get to the XP boot menu at startup.
3. Select safe mode.
4. Get in with any user id.
5. Go start -> run and type 'CMD'
6. Go to your WINDOWS/System32 directory.
7. Then type Copy CMD.EXE Login.scr (
make sure to backup Login.scr first)
8. Then type 'net user Administrator password'
You should get a successful message if it works, then you can login to the Administrator account with the password you assigned, though whoever had adminstrative access before won't know the password and therefor will be PISSED!.
Anyway, if it doesn't work and you get an error then restore your Login.scr file.
No clue if this works, let me know what happens if you try it.