The move from Google to start notifying it's users malware began when it's engineers noticed unusual activity coming in while doing routine maintenance at one of it's data centers. Damian Menscher, a security engineer at Google, wrote in a blog post, "After collaborating with security engineers at several companies that were sending this modified traffic, we determined that the computers exhibiting this behavior were infected with a particular strain of malicious software, or 'malware,'"
The nature of the malware sends traffic to Google from infected computers through multiple proxies. "We hope that by taking steps to notify users whose traffic is coming through these proxies, we can help them update their antivirus software and remove the infections," says Menscher.
If you don't see the alert from Google, it doesn't mean that your computer is completely clean. Google is currently targeting this specific piece of malware. If you do get the notice though, Google does provide a link instructing how to remove it.
Google already announced plans to start notifying users of it's Chrome browser with warnings about malicious downloads.
Google states that they make money when people use the internet, therefore they have a vested interest in the health of the net. Providing "healthcare" to the online community ensures it's continuing success.