If you haven't heard, Microsoft recently started making Microsoft Security Essentials (MSE) available through Microsoft Update. See this Microsoft article for details on when you're offered MSE. Basically, it's an optional update if you don't already have any anti-malware programs installed.
I like MSE a lot. I've used it since the beta and I recommend it to anyone who'll listen. It's the only anti-malware software I've ever used on my personal machines that doesn't regularly annoy me with pop-ups, thinly-veiled ads, and/or attempts to show me how well it's working. It's developed a good reputation among people who usually bash anything Microsoft does, and it's free. Check out Paul Thurrott's review of MSE from last year—he sums it up well.
At least one company is speaking against making MSE available through Microsoft update. Computerworld broke this story, and I don't have anything factual to add, so check it out. But that doesn't bother me. What does bother me is software getting installed without your explicit permission. I liked the beta of Safari 4 last year but as I kept it around for a while, I noticed that Apple's updated desperately wanted me to install iTunes and other software that I wasn't interested in. Every time Java updates I have to be careful to clear a check box or it'll install a browser toolbar or something. And there's some great software in Microsoft's Windows Live Essentials, but the minefield of uncheck-or-I'll-install it is aggravating. So part of me is opposed to MSE getting installed automatically.
But every virus-ridden PC out there increases the power of the botnets that send out spam and generally annoy the world. If you're the type of person running Windows without any anti-malware, you're either savvy enough to take care of yourself out in the wilds of the Internet, in which case you can figure out which check box to clear, or you're so clueless someone should really step in and help you. So part of me is in favor of MSE getting installed automatically.
I think, overall, having MSE installed on an "opt-out" basis is for the best for everyone. When your computer's infected, you don't just risk hurting yourself, you risk a lot of other people. There are plenty of companies out there installing software unless you hunt down their check boxes. At least in this case it could make the world a better place.