Don't know much on this but see if anything here: http://home.eunet.no/pnordahl/ntpasswd/ or http://www.petri.co.il/forgot_administrator_password.htm or http://en.allexperts.com/q/Windows-XP-3282/windows-xp-password-forgotten.htm
OphCrack. Best of all, it's free. Download, burn, and boot from the CD. http://ophcrack.sourceforge.net/download.php?type=livecd
before you try anything see if you can log on through the default admin account using safe mode. 75 percent of the time people don't bother setting an admin password and it leaves windows wide open if you boot up in safe mode. you can go and delete the password of any other account on your machine. hit f8 upon start up.