Bad version number in .class file
That could mean that the class file is from an younger version of Java, say 1.6 and the java program you are using is older and doesn't recognize the version number in the class file and gives that error message.
What version of java are you using to execute the class files?
Enter java -version to see what version. Here's what I see:
C:\Documents and Settings\Owner>java -version
java version "1.5.0_04"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.5.0_04-b05)
Java HotSpot(TM) Client VM (build 1.5.0_04-b05, mixed mode, sharing)