The Security Mentor

A study by Google of 4.5 million web pages found 450,000 which try to do "drive-by downloads", unauthorized installation of software on your machine.

This tells you much less than you might think.

First, how did they pick the pages to investigate? At random, or at random among frequently visited places, or what?

Second, just because it tries to do something malicious, that doesn't mean it will succeed. You improve your odds a lot by keeping up to date with security patches and running Firefox or Opera as your web browser. Antivirus software may help too.

Third, those pages aren't spread out evenly. They're concentrated in bad neighborhoods. Sites with illegal copies of copyrighted software are notorious, and a lot of "free games" are bait for malicious web pages.

The most interesting thing to me was that it's often not the fault of the people who put up the web page. The malicious content may be in ads, or visit counters, or other things that they don't directly control. The Firefox extensions AdBlock and NoScript make it harder for things like that to make it into your web browser.