as soon as you turn the machine on hit F4, F6, and F8 repeteadly ( really only one works but it varies per motherboard so I do all 3 ) - it will then ask what mode you wanna boot in - go in under safe mode ( with networking ) and you cna login as the Administrator

once logged in safe mode, go to Control Panel, then User Accounts - you can either A) delete the old account or B) attempt at changing it