Egy ideig működött, majd nem… A megoldás:
Open up an administrator command window (this is needed if you’re using Vista or Windows 7 with UAC enabled) and do:
assoc .jar=jarfileterm
ftype jarfileterm=”C:\Program Files\Java\jre7\bin\java.exe” -jar “%1” %*
In your case, you should replace the C:\Program Files\Java\jre7\bin\java.exe path with the one for your install of the jre.
Forrás: http://stackoverflow.com/questions/10446986/double-clicking-jar-file-does-not-open-command-prompt