Microsoft Security Essentials is out for beta. If you ask me, it can't hurt.
Here Chez Borepatch, I plan on loading it on the kid's computers, and the lovely and not-Linux-yet Mrs. Borepatch's Vista laptop.
I see that they don't yet have a version for Ubuntu Linux, so I'll have to hold off for myself ...
UPDATE 24 June 2009 14:53: That was quick. The beta program is closed now. If you didn't get it, you'll have to wait for the official release in September.