I have XP and have admin rights, so no-one else can download programmes. After my grandson downloaded a thing that promised to "improve the gameplaying performance" of the PC, and it was a porn thing. He didn't see any difference in the gameplay, so switched off the PC. In the morning i switched on the PC and there was a picture of a camcorder on the desktop. When I clicked on it, it showed a naked woman crossing and uncrossing her legs, and it had a dialler and tried to connect to the Internet. It took a lot of work to get rid of it as it kept coming back.
Now I have to OK anything downloaded.