In fact, about one out of every 14 programs downloaded by Windows users turns out to be malicious, . And even though Microsoft has a feature in its Internet Explorer browser designed to steer users away from unknown and potentially untrustworthy software, about 5 percent of users ignore the warnings and download malicious Trojan horse programs anyway.

Five years ago, it was pretty easy for criminals to sneak their code onto computers. There were plenty of browser bugs, and many users weren't very good at patching. But since then, the cat-and-mouse game of Internet security has evolved: Browsers have become more secure, and software makers can quickly and automatically push out patches when there's a known problem.

So increasingly, instead of hacking the browsers themselves, the bad guys try to hack the people using them. It's called social engineering, and it's a big problem these days. "The attackers have figured out that it's not that hard to get users to download Trojans," said Alex Stamos, a founding partner with Isec Partners, a security consultancy that's often called in to clean up the mess after companies have been hacked.

Social engineering is how the Koobface virus spreads on Facebook. Users get a message from a friend telling them to go and view a video. When they click on the link, they're then told that they need to download some sort of video playing software in order to watch. That software is actually a malicious program.

Social-engineering hackers also try to infect victims by hacking into Web pages and popping up fake antivirus warnings designed to look like messages from the operating system. Download these and you're infected. The criminals also use spam to send Trojans, and they will trick search engines into linking to malicious websites that look like they have interesting stories or video about hot news such as the royal wedding or the death of Osama bin Laden.