C# App deployment Framework detection

[MontgomeryBurns]

I created a Setup Project for my C# Application.
Though I selected

Download prerequisites from the component vendor's web site,

it happens that when I launch Setup.exe the Framework 3.5 SP1 setup is asked to be downloaded, even though Framework 3.5 SP1 is already installed! If I double click the .msi file my setup app runs immediately instead.
The same setup build, on some other PC works fine (it correctly detects the presence of the framework). One of the PC where it does not work correctly is the developer's machine.

Everything with Microsoft Visual Studio 2008 with Framework 3.5 SP1 (which is my app's prerequisite).

thanks to anyone who'll help.

[vcdebugger]

check if the Framework 3.5 SP1 is installed properly in the developer's machine where it is not getting installed properly.
Also since it works fine when you click on .msi file looks like you have not built the setup.exe properly, revive once again building it...