1 - Situation:
 
You're standing in front of a pc. It prompts for user/pass. You don't have an account, nor password.
 
2 - What we're going to do:
Ok now, shutdown and restart the pc. Just after the POST-procedure, before windows gets loaded, start pressing F8 multiple times, until you get a menu.
 
Select 'Safe Mode', and continue. Windows will now be loaded in safe mode.
 
Than, after a while, you'll probably see the well-known windows-xp login screen. Good news: there's an 'Administrator' account! And even greater news: most people don't know that this account exists, so you have a 99%-chance that it is a password-free account!
 
So, just login as 'Administrator', do what you want to do, and restart!
 
(Tip: you might want to overwrite other users' passwords, or create a new account for future access