The worst thing that could possibly happen to your computer: letting other people use it.
Wife: "I've got a new screensaver."
Me: "Oh yes?"
Wife: "Something popped-up while I was shopping and said I could have a free Valentine's screen-saver. So I did."
Wife: "It's really, really good. Look!"
* Switches on computer, it is like running through treacle. After ten minutes, we finally crawl onto the internet. The homepage is now www.superwazoosexsearch.cz *
Me: "Oh God."
Wife: "And there's another thing. Something's wrong with the computer. I bet you did it."