You compiled your jar with a JDK version superior to the JDK used to launch the jar file (you might have compiled with 1.7 and try to run it with 1.6).
2 solutions:
If you don’t use specific features of the JDK you use to compile, set its target to the previous version (in the above example add the flag -target 1.6 to you compiler options)
Upgrade the JDK used on the client.
Upgrade the jdk version. You can upgrade it by uninstalling the older version from control panel and installing newer version of jdk. Get the new version here http://www.oracle.com/technetwork/java/javase/downloads/index.html