How to disable the "Turn Off Computer" button in windows XP

Follow the given steps to disable Turn off Computer" button on the start menu:

To use this feature, you will need to be logged into your computer with administrative rights.
Click Start button and type regedit in Run option then press Enter for next.

Here locate the location to:
HKEY_CURRENT_USERSoftwareMicrosoftWindowsCurrentVersionPoliciesExplorer

Here in right side panel, right click to create a new DWORD value with the name NoClose.
  Now assign number 1 in value data box and close the registry editor.

Now close the registry editor and restart your computer after any changes to go into effect.

But next time, if you want to enable "Turn Off Computer" button on start menu then simply change the value of data box to 0 or delete NoClose DWORD item.