Just before you get the XP loading bar, start mashing F8 on your keyboard, and select to boot into "safe mode".
Sometimes the administrator account in safe mode isnt passworded, and you can just make yourself a new account from the administrator account here.
If the Administrator in safe mode is passworded, then use Hiren's BootCD
Burn a disk with that image, and boot from it.
In the "Passwords" section, there is a tool called "Active Password Changer". Use that and it will strip out the password from the account.