Bad for security

It's a bit depressing that Microsoft will enter the anti-virus market in some fashion. If they build Microsoft-quality anti-virus protection into their OS's, many more people will think they don't need to buy a better-than-Microsoft-quality anti-virus system. If Microsoft sells their anti-virus solution, it will make it harder for the big anti-virus companies to sell their software, and quality will go down. In both cases, the security of the Internet will decrease.