I'm a little unclear what you're asking. By mentioning integrity levels, it sounds like you might want to do something similar to described on Designing Applications to Run at a Low Integrity Level. But reading between the lines, I think what you're really asking is how to launch your application as the original non-elevated user.
There's no great way to do this in an InstallScript project (which I'm assuming you're using, due to the reference to TARGETDIR). A Basic MSI project leverages Windows Installer's multiple process architecture, and runs the UI non-elevated so long as the bootstrap doesn't elevate it; in this scenario the UI can then launch the program at the end as the original user because the context hasn't been lost. The only approach I could imagine for InstallScript would be to try to match this architecture, creating an external limited-user UI project which then installs or removes an internal elevated project silently. This is not likely to be trivial.
