If you cannot change screensaver because the tab is missing in Properties window, this is a sign you may be infected by Vundo trojan/ MS Juan ./
Also the wallpaper cannot be changed because the wallpaper tab is missing.
Also your screensaver may be changed to Windous “Blue screen”
Another sign are many pop-ups - with ads of fake antivirus programs.
Do not click on this ads! They all are scam!
Here is a good program made especially to fight Vundo virus: http://vundofix.atribune.org/
Vundofix is free. You can read about Vundofix program here: http://en.wikipedia.org/wiki/VundoFix
