The biggest installed base by some distance is in the US, but the UK (1.7 million users), Brazil (2.5 million users) and Germany (1.6 million users) have given the software a global presence of software now available in 27 languages, the company said. Russia and China are still some way behind in uptake.
In total, 27 million PCs have reported infections to Microsoft's Malware Protection Centre (MMPC), a relatively high proportion.
It costs nothing to download and is backed by a huge brand but has Security Essentials been disruptive to the paid antivirus market?
On first inspection, 31 million users is modest next to the total installed antivirus software base, and even more so against the roughly 1.5 billion PCs in use in the world at any one moment.
However, although not stellar outside the US, the growth rates look steady, and should easily double in the next year. It is pretty clear that Security Essentials is going to become a 100 million plus user community at some point in the next two years off the back of good US growth.
At that level it will have to be eating into the subscriptions of current antivirus giants, Symantec, Trend and McAfee. According to Opswat, 42 percent of the market is already free.
The standard response from most antivirus companies is that Security Essentials is a basic product, and others have tried to give away free security software (Avast and AVG in Europe), and remained modestly successful.
But these companies are not Microsoft and do not have its development clout. Already, the product has been upgraded once in a year and recently launched a small business version for up to 10 users. It is gradually expanding its range and improving the features on offer. Paid vendors will be nervous.
"By making Microsoft Security Essentials easy to get and easy to use, Microsoft hopes to encourage broader adoption of antivirus protection across the consumer audience, which in turn will help increase security across the entire Windows ecosystem," said Microsoft's Eric Foster.
See also: Hotmail gets new security features