Java APIs

Hi,

I am getting this "Java Virtual Machine Launcher" failed to load main-class whenever I try to open a jar file that I used to be able to open before.
I recently uninstall java 1.6 and install java 1.5. The jar files that I am trying to open were created by java 1.6. Would this be a compatability issue?

I searched online and could not find a convincing solution. Many suggested to install an adware or play around with the registry which I never think was necessary. Any ideas?

Thank you.