You have to create a Custom Action for that. That is an Installer-specific type of class that you then register as an action in the Setup project.
In a Custom Action, you can do anything you want, such as verify the existence of a registry key (which would indicate the presence of installed software), run EXEs, etc.
To find out more, check out these links:
- Tutorial: http://www.c-sharpcorner.com/UploadFile/87ad51/3/
- On MSDN: http://msdn.microsoft.com/en-us/library/aa368066(v=vs.85).aspx
Incidentally, Visual Studio 2012 and 2013 do no longer come with Setup project support; instead you can install the (free) InstallShield Limited Edition. But guess what: the InstallShield Limited Edition cannot execute custom action classes from .NET DLL projects, only as command-line EXEs. Just so you know that, when you upgrade to a more recent Visual Studio, you'll have some work to recreate your installer and modify your custom action classes.