While performing routine maintenance Google discovered unusual patterns of activity. They were able to determine this corresponded to a group of users that had been infected with spyware. Google has been showing a warning message at the top of search results to affected users.
Notification has already been shown to hundreds of thousands of users. They estimate that at least a couple million PCs running Windows are infected by this rogueware that has hundreds of variants. Google’s notifications will not cover a wide range of spyware strains, but only the specific ones that were detected by Google.
The best advice is to install security from a trusted brand like Kaspersky or McAfee, and never download any unknown file off of the internet.