How to get open a jarfile if none of them will open by double click or command prompt?

I have been having this problem for about 2 days now and ran into it because my Minecraft server keeps crashing when more than one person joins which i looked in the console when it happened and it said that the version of mcpc+ was old so i got the latest version of 1.6.4 and since then i couldn't open any.jar files at all i tried the registry to see if it was broke and it wasn't i also tried changing the default program to javaw.exe and it said it was unable to access the file then i tried just to reinstall java and nothing happened still when i double click on the new version of mcpc+ to unload all my server stuff i get a little loading icon for a split second then it disappears someone please help i just want to play with my friend on my server that i payed 40$ for