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.
More than 120,000 Windows-based computers running Internet Explorer 9 are infected in the U.S., researchers say.
The botnet has targeted at least 202 Web sites that serve 14 billion ad impressions. The botnet apparently accounted for 9 billion of them.
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.