How do I persuade a VS2005 msi to upgrade?

Question

I have a Windows service written in C# using VS2005.

The installation is via a wizard that calls msiexec to install the msi file also created using VS2005.

I am

Answer 1

One more thing in addition to mohlsen's answer (For Visual Studio 2008):

In order for your Primary Output (your EXE!) to upgrade properly, you must increment the FILE VERSION

This setting can be found in the Project Properties: Application Tab -> Assembly Information