I've been looking for this solution since I started playing with the Fireworks CS3 prototyping symbols:
I upgraded from Fireworks MX 2004 to Fireworks CS3. The upgrade set my default image editor to Photoshop. I tried unsuccessfully to make Fireworks the default image editor. Windows did not throw an error nor would it associate the application. So I dug into the registry and found this key:
The following keys were still pointing to my old Fireworks install:
After changing them to point to my Firworks CS3 install I successfully changed my default image editor back to Fireworks.