Did you try deleting the 'Extension' subkey, as explained in the link Bryan provided?
Because that might well be responsible for your problem.
Or try this:
Create a new Text file on you desktop, and paste the following bold text into it:
Save as Extension.reg
Doubleclick it, and answer 'yes' when asked whether you want to merge the contents of the file into the registry.
With a little bit of luck this ought to solve your problem.