Joke/Slidescreen is classified as a Joke.
Jokes are programs intended to confuse the user by displaying false information.
The Fortinet Anti-Spyware Research Team is currently in the process of creating a detailed description for this Spyware.
For additional information about Jokes, and how Fortinet classifies Spyware threats please see Fortinet's paper in PDF format:
Spyware Classification and Terms.