>This type of software will interecept the Setup and will take a snapshot of your system (directories, files, registry keys, etc). After the install, it will take another snapshot and will do the comparison.
>
>When the user wants to uninstall your software, he will use that Uninstaller software, wich will eliminate all the tracks of your app. And after that, the computer will a new one :)

This can be circumvented - put a signature not after the install, but after the 42nd launch of the app. Disable Save when demo period expires.

nb: 42 is arbitrary - a far lower number is still OK. It's just that I'm in the Arthur Dent phase again, reading The Guide to my daughter evening after evening :)