XP 

Since we are going to do all the Editing in Window Registry it is Recommended to Back Up the Registry before going Further. After you have Backed up your registry follow the Steps to Create your Hidden Account: 
First Goto Start -> Run -> Type regedit -> Enter
In the Left Menu goto,
HKEY_LOCAL_MACHINE\Software\Microsoft\WindowsNT\CurrentVersion\Winlogon\SpecialAccounts\UserList
In the Right pane, Right click -> New -> String Value
Right click on the new String Value and click Rename
Type the Name of the Account you want to hide.
Hit Enter then Right click on the String Value again and Change value to 0 which hides it. If you want it to be Visible to all Enter the Value 1.
Now Save and Exit the Registry and Logoff.
Goto welcome screen and Hit ctrl+alt+del twice to bring up Logon prompt
Type hidden Accounts name and password
Enjoy!!!