New to Java

Hi all

I have multiple JREs installed on my computer, which I need to keep.
I'm doing some development, and when I try to run the class, I get the following error:

Exception in thread "main" java.lang.UnsupportedClassVersionError: HelloWorldApp
 (Unsupported major.minor version 50.0)
        at java.lang.ClassLoader.defineClass0(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)

java -version shows:

java version "1.3.1_01"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.3.1_01)
Java HotSpot(TM) Client VM (build 1.3.1_01, mixed mode)

The reason I'm getting this error is because Eclipse is using the following JRE:

C:\Program Files\Java\jre6\bin>java -version
java version "1.6.0_17"
Java(TM) SE Runtime Environment (build 1.6.0_17-b04)
Java HotSpot(TM) Client VM (build 14.3-b01, mixed mode, sharing)

My Path environment variable is:

C:\Program Files\Java\jre6;C:\oracle\ora92\bin;C:\Program Files\Oracle\jre\1.3.1\bin;C:\Program Files\Oracle\jre\1.1.8\bin;%SystemRoot%\system32;%SystemRoot%;%SystemRoot%\System32\Wbem;C:\Program Files\Intel\DMIX;C:\Program Files\Gemalto\GAC

My CLASSPATH environment variable is:

C:\Program Files\Java\jre6;C:\Program Files\gemalto\gac\GATicket.jar;

How do I force my Windows XP system to use the latest JRE installed?

Thanks