Judging by the myriad of problems users are experiencing with Windows Defender, especially the "won't uninstall" problem which leaves you in a sort of "catch 22" situation, and the fact that it's known to be a poor alternative to it's rivals, I suggest we all give it a wide berth and instead use something else. I don't know of many forums (and I'm a member of many) which devote a whole forum to this weak anti-spyware offering from Microsoft.
I suspect many computer users are misled by the fact it's got the weight of the mighty Microsoft behind it "so it must be good". Obviouly not true.
And the "Live OneCare" launched by them as a "total" anti-malware and ant-virus solution has proved also to be disappointing.
The third-party security software developers need not fear that their established territory is under threat from Microsoft. Not judging by these two offerings at least.