The initial thread is here: http://www.thescripts.com/forum/thread336476.html

The answer is here: http://www.knowdotnet.com/articles/autouninstall.html

When you change the Major, Minor, or Revision version for your Setup Project, you should get a pop up saying "It is recommended that the ProductCode be changed if you change the version. Do you want to do this?". Say yes, as it will generate a UID for the ProductCode...