I find a solution who works for me (Win7 64bit) !
Use a Registry cleaner (ccleaner) and kill old file format entry !
Now restart association with a .AI file, because the way to .exe is in another place like other apps !
Try this:
- First, right click on your illustrator apps icon (desktop or Start menu) and choose "open the file location" and copy the dir ("C:\Program Files\Adobe\Adobe Illustrator CC 2014\Support Files\Contents\Windows" for me )
- Right clic on your .AI file and select "Open with", now click on "browse" to "C:\Program Files\Adobe\Adobe Illustrator CC 2014\Support Files\Contents\Windows" for 64bit system.
- "Windows" say "you've got problem with registry file association" the correction's automaticatly and the illustrator icon appear in the list of default apps.
- Select and valid him
Now, do the same for all your "unknown files type" with the good apps !