Tested on :Windows 7 /standard user /UAC on

Steps :
1. Uninstall all JREs. And remove the jusched.log (located in %TEMP%) file in temp dir.
2.Install the test JRE -6u18 b74 -in UI mode.
3. Import the registry file in your system.
http://javaweb.sfbay.sun.com/~ranjitl/deployment/setupfiles/RegSetup-6u20b73.reg
4. Open Java Control Panel goto Update Tab and click on the Update Now button
5. Update Notification should appear. Install the Update.

6u18 ->6u20 is working fine.

For 6u20 ->6u21 auto update testing this XML file is used.
http://sqindia.india.sun.com/disk17/deployment/pg152891/PIPTest/map.xml

Update baloon is coming but auto update of 6u20-> 6u21 is not happening because of the error 'Failed to download required installtion files.'