TweakUI for Windows XP allows you to show or Hide an user Account from the Welcome Screen. TweakUI can be downloaded from here:

Install TweakUI and run it. Click the menu Logon in the left pane and you'll see the list of User Accounts in the right-pane list box. Uncheck the respective entry which you want to hide from the Welcome Screen. To show the account, put a checkmark on the entry. Click OK and close TweakUI.