User password not known

I have just upgraded a PC that was using windows 2000 to XP. The PC was always being logged on as Administrator.

Ofcourse now that the PC is upgraded to XP the logon screen shows the user logons and doesnt allow you to select Administrator.

How can I login without the user password, or how do I change the pretty (but annoying) fast login screen to the logon screen where I can type in administrator.