Reason #1: The "Security Center" that Microsoft has put in their flagship OS is nothing more than a comfortable blanket for it's users. "Hmm. I have a Security Center in Windows. It must be secure now." Please. Here is why the "Security Center" sucks- it doesn't update 3rd party applications, at least ones that don't have deep pockets.
At work, we have Trend Micro OfficeScan for our virus protection. I don't know one way or the other if it is great software, or if it sucks, however, I know that updating the software through the Trend Micro OfficeScan GUI works just fine. And, if Microsoft can tell me that my virus definitions are out of date, then it certainly should have the ability to update them. Sounds normal to me. Anyway, here is a link to a screenshot of the "Security Center" on my desktop at work.
"Note, Windows does not detect all antivirus programs." Way to say, "if you haven't paid us money to fix it, we won't fix it!"