If you are doing a search on Google and a big yellow bar pops up at the top of the screen that warns you that “your computer appears to be infected,” then don’t think that’s one of those spam ads that try to sell you a fake malware removal service. Google has actually detected a major a massive influx of “unusual search traffic” that’s flowing through its data centers due to malware-infested Windows machines. As Google can detect whether your machine is one of the large number of compromised PCs that harbor this malicious piece of software, it can also warn you about it.
Google also provides those users with compromised machines a link to this Help Center article with instructions for detecting and deleting the malware. The infected machines all try to access Google’s servers through a small set of proxies. This makes it relatively easy for Google to positively identify them.
As Google’s Matt Cutts notes on Google+, the company is “trying this as an experiment to alert and protect consumers that we believe have infected machines.”