In WindOS 7 x64, MSU file type is associated with WUSA.exe under System32 folder.

I made a big mistake by changing it to Explorer.exe under Windows folder.

Now, I cannot revert back the file association permanently. The Always use the selected program to open this kind of file checkbox is disabled under Open with | Choose default program.

I tried to delete the registry entry (HKCR | .msu) but no luck.

Is there any fix for it (other of reinstalling Windows)?

1 Answer 1


The Windows Registry contains a list of all the file associations, specifically the "HKEY_CLASSES_ROOT" section. Deleting the entry will not fix the issue as that makes it not connected to anything. What I think you need to do is set the key(s) back to its original values.

BTW I exported the key for the .MSU key from my Windows 7 Home Premium computer which should still be at the default value. Should be able to plop this into Notepad and save it with a .REG and then run it:

Windows Registry Editor Version 5.00


@="Microsoft Update Standalone Package"





  • Tried that. Unfortunately, it didn't permanently changed the association and the Always use the selected program to open this kind of file checkbox is disabled. Mar 17, 2013 at 9:07
  • @vulcanraven: Try the extended registry export above and see if it helps.
    – Karan
    Mar 18, 2013 at 20:07
  • @Karan, still the same and the checkbox is disabled. Mar 19, 2013 at 11:32
  • @vulcanraven: You shouldn't need the Always use the selected program to open this kind of file checkbox since the registry edit ought to have set the association back to defaults. After importing did you reboot and then try running an MSU file? Odd it didn't work; I can't see anything else related to .MSU association in the registry.
    – Karan
    Mar 19, 2013 at 17:57
  • @Karan, yes I did and its still associated with explorer. Mar 20, 2013 at 9:02

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