Windows Explorer and My Computer will not launch

> I was messing with the XP Windows Registry and must have
> done something bad, because now I can no longer open
> Windows Explorer or any other application that opens
> directories.
> When I double click on My Computer or a short cut nothing
> happens. No error message, nothing. I can still access
> files through other application that still execute, i.e. MS
> Word.
> I have tried re-installing Windows XP, but no change. What
> did I do and how do I undo it?
>
>