Microsoft Security Essentials Is the Number 1 Anti-Virus in the World

Yeah...because people mostly get that downloaded through windows update,The anti virus itself is poor

I do not know how Opswat achieves their data about the programs installed on various PCs.

I will assume that their data are accurate. And that MSE "leads the pack".

And now... everything depends on how you interpret the statistics.
Here is my own interpretation:

15.3% of the scrutinized users have indeed installed MSE. Good.
But... that's all: they are not actually RUNNING the application. It's only installed. It is there. But it is not active.

Why it has been installed? Well, if it is not an un-removable component of the OS, I suppose it was installed for didactic purposes. For example, parents wants to show to their kids (or vice-versa) how an application should NOT perform. Or want to compare it with other similar apps.

Others installed it out of curiosity. And forgot to delete it. Or postponed the un-installing procedure.

On countless PCs, it might have been installed by the PC maker. It's sort of bloatware. And the average PC user does not bother to replace it with a better Security solution.

Other people downloaded it "by accident", after they visited Windows/Microsoft Update website(s).

Also, other guys are running Windows 8 in a virtual environment (researchers, hackers who study the OS, testers, a.s.o.). And Opswat wrongly concluded that MSE was "installed".

MSE: today is detected as "installed". Ans so what? What proves this?! After three weeks, or two months, MSE ends-up un-installed.

My conclusion? A statistic is like a very short skirt: it shows everything. But it does not show "the essential"...