You Might Be Infected
Adware has been around for a long time, but it's still going strong. And at least five million computers are infected with it.
Google and UC Berkley did a five month long study to determine how many computers are infected with adware. They replayed network requests of computers visiting Google websites to see if fake ads were injected onto those computers. In that time, 5.5% of the IP addresses that went to Google sites were infected with some kind of malware. Five-and-a-half percent may not sound like much, but that's over 5.3 million different IP addresses, and the real number is likely to be higher, since adware doesn't usually bother with bigger company websites in order to try to maintain secrecy.
In case you need a refresher on just what adware is, it's malware that injects ads onto websites users visit. When a user clicks on the ad, it sends money back to the hacker who put it in place. It may not sound all that terrible, but take a look at Superfish, the most famous adware out there and was recently discovered to be preinstalled on Lenovo laptops. In addition to sending money to people who put things on your computer that you don't want, it also breaks SSL protections for computers in the process to make sure its ads get through.
Adware often comes bundled with free software, including antivirus programs (ironically), compression utilities like WinZip, video codecs, major web browsers and pretty much anything else you can think of. It's so bad that Google stopped taking AdWords for free programs.