Skype preventing XP shut down

Skype prevents Windows XP from shut down on my friend's computer. I
click StartShut DownTurn Off Computer and XP closes half of the
running tray icon programs but then XP just keeps running like the shut
down request was never made. I have to do StartShut DownTurn Off a
second time to make it work.

If I manually shut down Skype.exe first the computer shuts down
correctly so I'm quite sure that Skype is the culprit. Any
registry-entry tweak to solve this without having to manually close
Skype prior to shut down? I've reduced the time delay in the registry
but that doesn't help. I need something like killtask to do the job but
I want it to happen automatically, not with a separate shut down icon,
that's why I'm looking for a registry type solution. Thanks.