Google has detected infected websites for a while, but now they're experimenting with detecting infected browsers, too. If you visit www.google.com and see a message at the top of your browser saying, 'Your computer appears to be infected,' it's got a very specific piece of malware that Google has detected, and you'll also see a link to fix the issue.
As computing moves from the desktop to the cloud, it's particularly important for companies like Google to keep control of malware. Hijacked machines can slow down and stop Web apps and slow email systems to a crawl. That hurts Google's bottom line, and ultimately it hurts consumers, too.