Setting which users appear on Windows XP's welcome screen

The "welcome screen" of Windows XP is the screen which shows a list of
available user accounts on the computer, allowing you to simply click on an
account to log in under that account. This is considerably less secure than
the traditional login method of having to actually type your username;
therefore, use of the welcome screen is generally deprecated by real computer
users. However, the welcome screen is required in order to use "Fast user
switching", which is actually a genuinely useful feature (it allows another
user to log into the computer without having to log off the current user
first), and so use of the welcome screen may be understandable on a computer
which is used by multiple people who might wish to use accounts concurrently.
If you enable the welcome screen, however, you'll then want to know how you
can control what user accounts are shown on the welcome screen. Luckily, this
isn't too difficult to do. Oddly enough, however, Windows XP seems to have no
built-in tool to allow you to do this, so you must do it through a
manually-changed registry setting.

The registry key you are looking for is:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon\
  SpecialAccounts\UserList

Within this registry key, you can simply create a DWORD value with the name
of the user account you want to show or hide on the welcome screen. Then,
setting this DWORD value to a 0 will prevent the account from showing up on
the welcome screen, while setting the value to a 1 will cause the account to
be shown on the welcome screen. For example, if you want to hide the
Administrator account from the welcome screen, create a DWORD value called
"Administrator", and set it to a value of 0. (Actually, the default value for
a new value in the registry is 0 anyway, so if you just created this as a new
value, you don't need to change it.)

If you're at Windows XP's welcome screen and you want to log in under an
account that's not shown, press Ctrl-Alt-Del twice to bring up the old-style
login screen which requires you to type both your username and password.