I'm on Windows XP and can't open Illustrator .AI or .EPS files by double clicking them. When I do I get the error "...is not a valid Win32 application". I must go to file>open or drag the file icon to the AI icon. I checked some other threads, but had no luck. I did try going to control panel>folder options>file types>opens with. But that did not work. Photoshop and Acrobat files open fine when I double click them.
As I wrote many times, the best cure in such situations is to entirely delete any existing references to a file type or program from that list, then create the rules again from scratch. Nothing as refreshing as starting with a clean slate. In addition, check, whether other programs are having a hold on certain associations and/or are simply flooding your context menu with nonsense stuiff. Tools such as Nero burning software, various packer tools and a colorful array of other software have the bad habit of doing that...