Microsoft's next version of Security Essentials is actually a pretty great update from its already-great predecessor. Microsoft's option has always been adequate at finding new malware without definitions, but the addition of an heuristic engine bumps its power up quite a bit. It may be subject to a few more false positives, but you're much less likely to get hit with malware than ever before.
It also includes some Windows Firewall integration that lets you tweak the Firewall from inside Security Essentials, as well as a network inspection feature that can inspect traffic as you browse, which is pretty neat. All in all, if you're using Microsoft Security Essentials (and why wouldn't you be?), you'll want to grab this update. Hit the link below to manually download and install the update.