Run Ophcrack on it. There's a live CD you can boot to and crack the password that way. http://ophcrack.sourceforge.net/
Lifehacker did a good walk-through on how to crack the password with Ophcrack. http://lifehacker.com/232963/screenshot-tour-how-to-crack-a-windows-password-with-ophcrack-live-cd
If you're impatient, run Offline NT Password & Registry Editor. http://home.eunet.no/pnordahl/ntpasswd/
Download the ISO, burn, and boot. It will drop you into DOS, where you can clear the admin password.
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