There's no need to go overboard on it and worry about security and such. Just do something basic like what electronin suggested. People just need a push to register if they're honest ... if they aren't, they're going to defeat whatever you put into place anyway.
A simple way to overcome the uninstall/reinstall thing is to write the value into the registry. Then it's always there, regardless of uninstalling/installing processes. The average user isn't going to spend time trolling their registry looking for the key.
A simple way to overcome the uninstall/reinstall thing is to write the value into the registry. Then it's always there, regardless of uninstalling/installing processes. The average user isn't going to spend time trolling their registry looking for the key.