The company said that it noticed unusual behavior on its servers and was able to trace it back to malicious software activity and malware that was installed on client PCs. Users with infected PCs now see a prominent warning indicating that their PC houses an unwelcome guest.
Google said that computers infected with the malware wills end traffic to Google through proxies. The company did not provide details about the malware, but noted that it may have installed on PCs via fake antivirus software promotions, which have circulating on the Internet for several months. Google said that it estimates about a "couple million" of PCs to be infected and that "hundreds of thousands" of users have been warned.
The company also said that users should be careful about malware authors copying Google's warning window and tricking them into further malware downloads.