You will need to enter the command net user Administrator /active:yes at an elevated command prompt. Click Start, type in cmd, then when it finds Command Prompt right-click and choose Run as Administrator. You will be prompted for username (Administrator) and password (blank), UNLESS the User Account Control has been set to Never Notify, and assuming the Administrator account still has a blank password.
Then restart and log in as Administrator with no password. If you're successful, you can go to Start > Computer Management > Local Users and Groups (Windows Pro only) and reset the password on your other account as well as putting in a password for the Administrator account.
Then you can hide the Administrator account, this time with the "no" option
net user Administrator /active:no
Steve