Google warns of 'drive by downloads'

The search engine giant trained its Web crawling software on billions of Web addresses over the past year looking for malicious pages that tried to attack their visitors. They found more than 3 million of them, meaning that about one in 1,000 Web pages is malicious, according to Neils Provos, a senior staff software engineer with Google.

These Web-based attacks, called "drive-by downloads" by security experts, have become much more common in recent years as firewalls and better security practices by Microsoft have made it harder for worms and viruses to directly attack computers