New to Java

Source not found: The source attachment does not contain the source for the file ClassLoader.Class

Hey,

I'm running eclipse Version: 3.2.2 Build id: M20070212-1330, on a Vista Home Premium Toshiba laptop. Everything was working fine before i installed the Java JDK 5 and JDK 6. Now all of sudden when i try to run one my programs that worked in the past i get the following error:

java.lang.UnsupportedClassVersionError: Bad version number in .class file
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClass(Unknown Source)
at java.security.SecureClassLoader.defineClass(Unknown Source)
at java.net.URLClassLoader.defineClass(Unknown Source)
at java.net.URLClassLoader.access$100(Unknown Source)
at java.net.URLClassLoader$1.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClassInternal(Unknown Source)
Exception in thread "main"

I'm not sure where to go from here...I've looked up a few articles that mention going into some system.ini files...etc... to make sure the variable are ok but i feel like i'm getting in over my head.

Thanks in advance for your thoughts,

Struggling to Run