Depends on the operating system..if you have XP Pro then go to Start, Run and enter
GPEDIT.MSC Go to User Configuration, Administrative Templates,
Control Panel, Display. In the right pane, locate the entry for Remove
Display in Control Panel. Double click this entry. It should be set to
Disabled or Not Configured.
If you're running XP Home, or if GPEDIT shows that this policy is
Disabled or Not Configured go to Start, Run and enter REGEDIT Navigate
HKEY CURRENT USER\Software\Microsoft\Windows\CurrentVersion\Policies\Syst
Look in the right pane for a value called NoDispCPL. If present, double
click it and set its value to 0 (zero). If its not there, right click
in the right pane and select New, DWord value. Name the value NoDispCPL
and leave its value at 0.
This should overwrite any changes you have made with some third party software like TweakUI.