If you are using the "Domain Style" Logon type then all you have to do is logon to the account Administrator as you would normally. However, If you are using the Welcome Screen Logon System all you need to do is Hold CTRL+ALT and then press DELETE twice at the Welcome Screen. Now you can enter the username as Administrator and the password you created for the Administrator account. If you don't Remember creating an Administrator Password, or just forgot it, see How to change the main Administrators Password.