Well, a quick search at sourceforge.net (a reputable open-source software site) revealed this:
http://sourceforge.net/projects/daves-av-ss/
It looks like it'll do what you want. I personally can't vouch for it as I haven't even downloaded it, but you might try it out. Also, the initial search I did on that site came back with over 1000 results, so you might do a search there.
And please don't do this by Google. You'll end up downloading spyware; anything marked "screen saver" from Google is probably trash.
JBot