While locking an XP system is always a good idea if you plan to leave a system unattended for some period of time, it can also cause issues for users, especially if the PC will be publicly accessible. For example, imagine an XP Pro system that is configured as an information kiosk in a business, or even a system at an Internet café. If random users chose to lock the system (by pressing Ctrl+Alt+Delete and clicking the Lock Computer button), an administrator would need to manually unlock the system. To avoid this, the Lock Computer button can be disabled.
To disable the Lock Computer button, open Regedit and browse to HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Policies\
System and HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\Policies\
System and create a new REG_DWORD value in each called DisableLockWorkstation. Setting this value to 0 will allow the Lock Computer button to be used, while 1 will disable it.