If you try unpopular's answer:
For Windows 7:
- Start "Control Panel"
- Click "Default Programs"
- Click "Associate a file type or protocol with a specific program"
- Double click
.jar
- Browse
C:\Program Files\Java\jre7\bin\javaw.exe
- Click the button Open
- Click the button OK
And jar files still fail to open (in my case it was like I never double clicked):
open the Command Prompt (to be safe with admin rights enabled) and type the following commands:
java -version
This should return a version so you can safely assume java is installed.
Then run
java -jar "PATHTOFILE\FILENAME.JAR"
Read through the output generated. You may discover an error message.