Problems with "File Association" in Windows 7 64-bit

Symptom: + Associate program with file extension does not work. Program selected does not appear in the Open With window.
+ Default Program also does not work or it ignores all changes.

Problem: The program that you're pointing to isn't registered correctly.

Solution:

In regedit: Navigate to Computer\HKEY_CLASSES_ROOT\Applications and find your .exe name. 

Navigate under its name to shell>open>command. In the Default change its location to the actual location of the executable, hit okay and then try and reassociate the file type as you normally would.

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http://answers.microsoft.com/en-us/windows/forum/windows_7-files/problems-with-file-association-in-windows-7-64-bit/8a84fcec-22df-4942-8e35-d98dbe96e327