Hopefully this hasn't happened to you in a long time, but how about the last time you used a friend's PC to discover he had installed three different browser toolbars on top of each other and that Google queries were being filtered and redirected to a (usually awful looking) third-party ripoff site.
Recently Google discovered unusual patterns of activity while performing routine maintenance on one of their data centers. Looking more deeply into the search traffic coming in, they were able to determine this corresponded to a group of users that had been infected with malware. Since yesterday, Google has been showing a warning message at the top of search results to affected users (see the screenshot below).
According to the search giant, the notification has already been shown to hundreds of thousands of users, an impressive figure for just 24 hours. They estimate that at least a couple million PCs running Windows are infected by this "fake AV" software (scam antivirus, also known as scareware) that has hundreds of variants. In other words, Google's notifications will not cover a wide range of malware strains, but only the specific ones using certain proxy redirects that were detected by Google engineers in the aforementioned data center maintenance.
Anti-malware tools are actively discussed and recommended on TechSpot. If you want to make sure your PC is safe, a good place to start if you run Windows is to download Microsoft's own Security Essentials software and Malwarebytes Antimalware, both free tools. You will also find personalized malware removal help in our forums courtesy of our kick-ass contributors Broni and Bobbye.