Question : Windows XP Pro Users cannot use Log off or Shut Down button

Recently a few users of XP Pro SP3 had been unable to shut down, restart or log off windows XP. In other words, if someone logs on and off, the pc is then unable to log off, restart or shut down. When log off/restart/turn off is pressed, the log off/turn off box disappears as it would normally but nothing happens and the computer runs normally. Task manager is, however, unaffected by this and still has the power to do these
actions, as is the "Turn off Computer" button on the Welcome Screen.

Download User Profile Hive Cleanup Service adn install it, does now work.

Any idea how to fix it ?

Answer : Windows XP Pro Users cannot use Log off or Shut Down button

Try the good old tedious troubleshooting method. Open msconfig and disable all startup items and see if the button works. If the button works then try enabling each startup items. I would disable non-microsoft services as well.  You said a couple of xp users but you didn't say whether it was on the same xp machine or different xp machines. If this was in multiple machines, I would guess that you install some kind of application or windows update on all of these machines. I would uninstall those updates/apps and try then.