It looks like you were using the default administrator account. Once you create a new admin account, the default one disappears from the login page.
How to fix it:
Vista:
My Computer (right click) -> Manage -> Local Users and Groups -> Users -> Administrator (right click) -> Properties. unclick disable account
XP:
download tweak ui here
Then Open TweakUI and goto the Logon option in the left pane. Check the option to show administrator on welcome screen. Close TweakUI, logoff and see if the welcome screen lists the administrator login. Other disabled accounts can be recovered this way also.