Ident and remove an Admin Pswd

You buy a new laptop instal vista and internet -but not an admin pswd (because frankly you dont expect to need one for a home use laptop), and pop out for lunch, leaving it at home and lo an behold you return to find someone with a key to your appartment has let themselves in and f...ed with your laptop and installed an admin password locking you out of what ever the fuck they are upto.

How the hell do you identify the PSWD so you can remove it?