Windows Live Onecare - The Conspiracy

You know about Windows Live Onecare, right? It's Microsoft's antivirus product that does some other things and it is a pretty polished suite. But, a thought struck me a little while ago. Why is Microsoft selling an antivirus suite for their own operating system? They must be making vulnerabilities in their OS just to sell another product. You see? It's a conspiracy! Ok, not really... But you can't help but wonder...