Fake updates. More and more I am seeing computers infected with software installed by fake updates. You are browsing the Internet and suddenly you are on an official looking page saying you need to update your Flash Player or your Chrome or Firefox browser. If you do the update several really annoying programs are installed that then modify your browser so you get sent to places that install even more of this crap.
The best thing to do is never do an update from something that shows up in your browser. Chrome and Firefox update automatically in the background with little intervention from the user. Flash will notify you with a popup by the clock that you need an update and current versions can be set to also automatically update without user intervention.
Here is an article on the ZD Net site talking about the same thing and showing some images. Go check it out for additional info.