I have to say that I am amazed how often this issue comes up and how confusing it is for some users. Why would you expect a purchase to cause a download? Is it something you've experienced somewhere else?
We may have to re-evaluate the flow, but I think that changing things might also cause problems. I don't think most people would expect a purchase of something to cause a download. Maybe I'm wrong.
What you want is a simple little stub executable
At it's most simple, it's a set of three queries:
1. Check if the version of Media Center the licence is for is installed- Read licence version from file, and pull the current install path from the registry, and query for it's existance.
2. Download and trigger the installer if not.
3. Wait and then restore licence when installer has finished.
TBQH, I'd probably change the way you do things very slightly, so the main download points to this stub installer, and so it'd work a little like this:
1. Download installer stub.
2. Select either install trial version, or enter licence code.
3. Download main installer and run.
4. Either restore the licence entered, or leave as trial.
I don't think this would be much work (Famous last words....), and it would IMHO solve a lot of confusion.
It also has the added advantage that you can always point the installer download to the latest version, providing an easy way to ensure that a user always has the latest stable version installed- Just re-run the installer and it'll figure out the rest
-Leezer-