NEVER EVER manually change a extension of a file unless you know what you're doing! This is the first step in destroying your PC!
1) Open My Computer
2) Click Tools>Folder Options
3) Goto File Types
4) find .TGA on the list (It should be there, if not, disregard this method, and move to step 7!)
5) click on the .TGA file ONCE and press the DELETE button. This will delete any binds to this file type.
6) Now, close all the windows, go back to your screenshot and try to open it. It should ask which program you want to view it with. You can try using IE Explorer, that should be able to view it.
7) If you want to edit/view the file the easiest, get Paint Shop Pro (
http://www.jasc.com) or Photoshop (
http://www.adobe.com)