QUOTE
Windows Defender has been slated in a new test that found it could detect barely half of the malware thrown at it during the last year...in full scanning mode the anti-malware scanner could only pick up 53.3 percent of an unspecified list of malware threats thrown at it during 2006, with the quick scan scoring a detection rate of under half...
techworld.com

Just a few weeks ago Webroot reported that Vista’s Defender stops only 16% of spyware.