"That's the big problem with this tool - the majority of the other products tested includes such protection features, so they are performing better in our tests. And we expect that they are performing better in the 'real world' as well, which is the focus of our tests."
An individual user's exposure to a zero-day attack will depend on a number of factors, including the range of applications used and how assiduously a PC is patched.
As Marx noted, MSE is a free product - many of the rival suites charge upwards of £20 ($33) a year for a license. However, the dividing line isn't necessarily whether a product is free or not; several rival products offered in free versions did better than MSE. It is possible that free programs now need to include a wider range of detection features than they might have done in the past.
Version 2.0 of MSE was launched in December 2010 and anecdotal evidence suggests it has only enhanced the program's huge popularity. By September 2010, the software was said by Microsoft to have been installed on 31 million PCs globally, including 1.7 million in the UK.
The most interesting message of these tests is that a product can drop in effectiveness quite quickly, before in all likelihood rising again as a new version appears that adds new security elements.