According to research done by Google, 1 in 10 web pages contain malicious code that could be used to infect a user's PC.  This based on a survey 4.5 million sites out of the billions crawled regularly by Google's web search spider.  In that number, around 450,000 were capable of "drive-by-downloads", sites that install malicious code without the user knowing.  Another 700,000 pages were considered capable of compromising a PC via other methods.

The research being done by Google stands a chance of not only indexing dangerous sites, but begin to paint a picture of how hackers are trying to compromise systems, what trends are emerging, and hopefully find ways to proactively block and counteract such attacks.

Could Google be the Anti-Virus company of the Internet?