First reboot Windows in safe mode by re-starting the computer and pressing F8 repeated as the computer starts up. Then (in safe mode) click Start and then click Run. In the open box type "control userpasswords2" without the quotes - I have just used quotes to differentiate what you have to type. You will now have access to all the user accounts, including the administrators account and will be able to reset the lost password.
Just click the administrators user account, and then click Reset Password.
You will need to add a new password in the New password and the Confirm new password boxes, and confirm by clicking OK.
All done, you have recovered the lost administrators password!