How to disable the Turn Off Computer button in windows XP

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.