Jar files are simply zip files where the very first file in the archive is 
META-INF\MANIFEST.MF. This file contains things like which class in the 
Main-Class, and which classes are java beans.

If you open Winzip from the start menu, then you can open the JAR file using 
the Winzip open button.

Or, in Windows, use the JAR tool, JAR.EXE, which should be somewhere on your 
machine if you have installled the Sun JDK. Hopefully JAR.EXE is in your 
PATH too...

Entering JAR at the dos command prompt will give you a list of options.

For example:
jar xf xxxxxxxx.jar

should extract all the .class files if you need them extracted for some 
reason.

If you have installed the Sun Jave Run Time Environment (RTE), then Windows 
will automatically associate file type .JAR with 'Executable Jar File', and 
it will run JAVAW.exe for you when you double click on a jar.

Hope this helps.