2 options - the admin password is set when you install windows. i.e. if you installed XP, then you specified the admin pass then. If thwe machine came pre-installed with XP, then you could phone the tech support for the company you bought it off & ask them what it is. If this doesn't appeal, here's a utility which will let you reset it:
http://home.eunet.no/~pnordahl/ntpasswd/