You need the admin password. You can try the normal ways
try Administrator with no password
Administrator with password as the password
Administrator with the computer's name as the password, if you know it. If not you can maybe see it from a computer on your network
Try phoning your IT company up and asking them for the admin password, then you can get into your PC. Apart from that you'd have to get a boot CD that can hack your password, or take it back.
Even if you do log in you will not be able to connect your computer back onto your domain without the correct privilages.