By default, the Windows XP Welcome Screen displays all the configured account names (unless there's only one account without password), with the exception of the Administrator account. On Windows XP Professional, it is possible to display the old-style Windows 2000 logon box for the current session, allowing you to type in a username. This is done by pressing Control-Alt-Delete twice, or by holding down Control and Alt and then hitting Delete twice.
Next time you log off or reboot, you will be presented the Welcome Screen again.
The Control-Alt-Delete trick works on Windows XP Home Edition too, but there is serves no purpose as you will not be allowed to enter the Administrator account; you must boot into Safe Mode for that.