doen a quick search, and first of, its anti spyware, not anti virus, so thats why you wouldnt have found it on any of the comparison sites.
sources:
http://www.adwarereport.com/mt/archives/000091.html
http://www.download.com/Microsoft-Windows-Defender/3000-12771_4-10353597.html
http://www.pcmag.com/article2/0,2817,1926597,00.asp
admittedly, it doesnt get as bad reviews as i had previously suspected, but its certainly not the cream of the crop.