Ultimately, Google wants to make as iron-clad as possible the relationship between the metrics it provides to advertisers and the actual number of people who are viewing its ads.
The botnet has targeted at least 202 Web sites that serve 14 billion ad impressions. The botnet apparently accounted for 9 billion of them.
More than 120,000 Windows-based computers running Internet Explorer 9 are infected in the U.S., researchers say.
Microsoft is investigating a researcher's assertion that all versions of the Web browser are vulnerable to a flaw that allows attackers to track cursor movements on the screen, even if the browser window isn't in use.