AV-test.org tested MSE against 3,200 virii.
Considering there have been over 5,000 since 1995 anyone with sense should see their attempts as woefully failing.
I haven't verified it, but from quick perusing it seems teh current virii database at symantec is around 90,000 virii large.
To date, M$ has released 76 security updates for XP.
That makes a total of 86,700 odd virii that MSE has not been tested against and that you are vulnurable to.
AV-Comparatives test with 22,000 samples.
I'm getting rather sick of "website this" and "website that" comparisons because honestly, they are purely wasting your time.
If they had even the remotest clue on what they were doing, they'd know that a good 90% of those 90,000 odd virii are still a threat today.
If we are not running a suitable AV, we are susceptible to virii that were written for windows 95 still.
To anyone who is considering a switch in AV's; Keep your old AV installed for a few months or scan regularly with some additional online scanners.