The built-in Administrator account is the most powerful user account on a Windows computer, and therefore the most dangerous if it should fall into the wrong hands. Here are a couple of things you can do to minimize the dangers posed by this account:
1. Create a separate admin account for yourself, add it to the Administrators local group on the machine, and never use the built-in Administrators account.
2. Then, either disable the built-in Administrators account or give it a password that's reeeeeaaaaly long like 100 characters or so (127 max) and write this down somewhere and seal it in a safe or something.
Cheers, Mitch Tulloch, MVP