If you don’t know all the login names for a WinXP computer (and probably most other versions of Windows), you can find them easily. (Of course, you still have to know the password…)
Press Win-R (or Start, Run) and type CMD, then press Enter. The Command window appears (used to be called the DOS window).
Type: Net user, and press Enter.
You will see all the User Accounts that have been defined (in Control Panel, User Accounts) on this computer.