Google found this link to a possible fix for your problem.

http://www.pctools.com/forum/archive...p/t-29720.html

Do not try to manually edit your registry, simply copy and paste the text into a
notepad file, save it as "x.reg", and then merge it into your registry.

Be sure that the first line reads:
Windows Registry Editor Version 5.00

[HKEY_CLASSES_ROOT\Folder\shell\open\ddeexec\topic]
@="AppProperties"

And there's a space after the last line.

"wernertk" <wernertk@discussions.microsoft.com> wrote in message
news:9E5AB610-A919-4A29-9BB5-CB2A413505C5@microsoft.com...
> EDIT:
>
> Forgot to add something. Before I used the regkey from
> windowsxp.mvps.org/ie/905915.htm, I got an error message "parameter is
> incorrect" when pressing the WinKey+E and no Windows Explorer. After the
> regkey, I got the above error message, and still no explorer.