Look up the file extension at
http://filext.com/ to find the program name which processes that file type.
If you have that program on your system, right click the file name (in Windows Explorer), click "Open With", then tell Windows what program to use.
However, this is an error message so something has gone wrong on your system. Possibly you deleted the program without using the Uninstall process or Uninstall may have failed to remove all the associated files. Or your Registry may be corrupted. If in doubt, post the file type here and let us help.