Windows 7 - Unable to open programs

Try the .exe download fix from this site.

File association fixes for Windows Vista
http://www.winhelponline.com/articles/105/1/File-association-fixes-for-Windows-Vista.html

See what this article mentions about a Registry Key called "UserChoice ", see if you can adapt that information and apply it where relevant to fix your problem.

Restore the .exe file association in Windows Vista, after incorrectly associating it with another application
http://www.winhelponline.com/articles/165/1/Restore-the-exe-file-association-in-Windows-Vista-after-incorrectly-associating-it-with-another-application.html

If this has only just begun to happen maybe you can use System restore to correct the problem.

How to repair the operating system and how to restore the operating system configuration to an earlier point in time in Windows Vista
http://support.microsoft.com/kb/936212/#appliesto

Can you do anything from a Command Prompt or System Restore accessed via the Startup Repair options?

Try typing ASSOC .EXE=exefile at the Command Prompt

ASSOC
Display or change the association between a file extension and a fileType
http://ss64.com/nt/assoc.html

Startup Repair: frequently asked questions
http://windowshelp.microsoft.com/Windows/en-US/Help/5c59f8c1-b0d1-4f1a-af55-74f3922f3f351033.mspx#EHF

How to use the Command Prompt in the Vista Windows Recovery Environment
http://www.bleepingcomputer.com/tutorials/tutorial147.html

Using System Restore from the Vista Windows Recovery Environment
http://www.bleepingcomputer.com/tutorials/tutorial142.html