Microsoft just rolled out Security Essentials 2 -- and it's even better.
The update includes a new, heuristic-powered anti-malware protection engine, better integration with the Windows Firewall and Internet Explorer, faster overall performance, and newly added protections against network-based exploits (but only for Windows Vista and 7 users).
Needless to say, if you're already using MSE, grabbing the update is a no-brainer. (It's available from the Security Essentials site, though the latter currently makes no mention of version 2.)
And if you're not using MSE? Well, I've made no secret of the fact that I think paying for security software is unnecessary, even for small businesses. (In fact, Security Essentials is now available for businesses with up to 10 PCs.) This may well be all you need.