What you'll need is an uninstall.bat file with the following in it:
@echo off
msiexec /x {FC5C0E0A-DAEA-4B54-9421-AB0E3F26C6AD}
where that GUID would be the ProductCode of the app to uninstall. (Don't just use the one shown above...it's just an example)
Just add the .bat file to your setup project and have it placed wherever you want during install.