Note: Defender has an advanced option to use heuristics to detect potentially harmful or unwanted behavior by software that has not been analyzed for risks. Heuristics allows Defender to detect potentially malicious activities for files that are not included in its malware definitions. The disadvantage to this feature is that it can potentially increase the chances that a non-malicious program is flagged as malicious. With heuristics, there is always a potential risk for a "False Positive" when the heuristic analysis flags a file as suspicious or infected that contains no malware.
Undeniably true, and yet interestingly, I've been using Defender all the way through its beta phase up to the present and it has never registered a single false positive response on my computer. (Actually, it's never found anything at all, but that's another story!) Although it doesn't have a good reputation for detecting malware as a scanner, I keep it on board because of its option to report significant system changes, which option I always keep switched on. (Yes, I know there are alternative programs that do this, but I feel comfortable with Defender after all this time.)
Windows XP Home SP2; AVG 7.5 Internet Security Suite (AV/AS r.t.p, and firewall); Windows Defender (r.t.p on); SuperAntispyware Free; a-squared Free 18.104.22.168; Spybot 1.4 (Immunised, but no Tea-timer); AdAware SE Free; AVG Anti-Rootkit Free; Spywareblaster; MVPS Hosts file (with HostsMan); McAfee Site Advisor.