Whenever I have a new InstallShield installation to write, I often copy an existing installation if it's close enough to the original.  That way I don't have to start from scratch.  I just change the installation as I need and I'm good to go.

Recently, I had a 2009 version of a product that I needed to write a Basic MSI installation for.  So, I copied the 2008 version of the MSI project and made the necessary changes to update it to the 2009 version.  However, I forgot to change one thing.  The Product code.  Can you guess what happened?

Well, if the 2008 version was already installed on the machine, when the 2009 installation was run InstallShield thought it was performing an upgrade instead of a new installation.  However, the 2008 and 2009 versions were different products, so this was a problem.  When I learned of this, I immediately knew what to do.  I just changed the Product code in the 2009 installer, rebuilt it, and the problem was fixed.

So, if you are like me and like to copy installation projects and then update them, remember your InstallShield training and change the Product code.  You'll save yourself some trouble.