For anyone that has accidentally clicked on an apllication, clicked "open with" and set it to open with another application by default. Here is how to fix it.
If you are lucky enough to still have a web browser open or some way of transferring files then put this file on your computer and run it. It will remove the default association for exe files.
http://craig.mayhew.io/uploads/2011/03/exe.reg
If you aren't lucky enough to be able to get this file to your damaged computer then you need to follow the steps in
this Microsoft article.
Windows 7 Registry File open with