The software *let see the screen*.

When you activate a Screen Saver, you *can't see the screen* because is covered by the screen saver.

Try this:
Let the computer unattended in a trade show or in your office (for your clients) with a movie of your company. Most of people will interrupt the movie to activate another application.

Why not just set a hot corner for the screen saver of your choice and enable the "Password Required" option? Same effect, no cost.