Reset the Administrator Password

In XP, click Start, click Run (in Vista, just use the Search field), enter control userpasswords2 (don't forget the 2 at the end).
In the dialog that appears is a panel titled Password for Administrator. Click the Reset Password button in this panel and assign a new password that you'll remember but that a malefactor or a virus won't guess.
If that button is disabled, check the box at the top that says, Users must enter a user name and password to use this computer. That will enable the button. After changing the Administrator password, close the main User Accounts dialog by clicking Cancel rather than OK. That way, your change to the "Users must enter..." check box won't actually change anything.