I just published a stand-alone Java Decompiler GUI (based on Jad) which you can get from Util Java Decompiler (JAD based) v1.0
This is a Windows based .NET 4.0 application, which supports the drag n'drop of *.jar files.
It doesn't integrate with Window Explorer, but since this is based on a simple C# script, and the code is Open Source, maybe you could add that feature :)
Another idea would be to also add support for the other free java decompilers.
