Microsoft officially entering the anti-virus industry means the incumbent anti-virus vendors will have to evolve, or die.
Think about it, folks! The AV industry has stagnated over eighteen years because of their reliance on the Addictive Update Model. AV software hasn't significantly advanced in all of this time because users won't buy better products.
A Microsoft anti-virus product is going to penetrate the AV market like Internet Explorer did. It'll be bundled with later versions of Microsoft operating systems, making current AV software obsolete. AV vendors will finally have to release better AV products - products that can catch viruses before the fact - just to stay competitive.
If the incumbent AV vendors respond to this threat to their bottom line, AV security will dramatically improve. About time, too.