There could be a real simple solution to this. I was forced at work today to find out how to dis-associate an ext to whatever app it's has been associated to because a user tried to open something they shouldn't have doen and clicked 'yes, always use this' crappy feature of windows.
If you open up cmd prompt (DOS window), try this:
C:\> assoc .html
Returns the app associated with that extension - in my case:
C:\> assoc .html
.html=K-Meleon.HTML
Now, with a bit of messing, to clear an association using above example:
C:\> assoc .html=
That's now cleared!
To assign .html to K-Meleon:
C:\> assoc .html= K-Meleon.HTML
&
C:\> assoc .htm= K-Meleon.HTML
!
Nick