New Anti-Malware available from Microsoft
And it's free! And it doesn't look like a total waste of time (unlike previous Microsoft attempts at this).
Check it out: http://www.microsoft.com/security_essentials
I've tried it on a few virtual machines, and it seems good. The proof is in the details, of course, and there's no way to know yet whether it can replace other comparable tools, but the price is right!