I have been using Microsoft Security Essentials since it was in beta. It does a good job, with very low overhead, and high rate of accuracy, including malware and Trojans.
If you use Security Essentials, keep your workstation patched, and use multiple browsers with ad blocking software, you can feel pretty secure running Windows.
Microsoft Security Essentials
Recommended Firefox Add-Ons – AdBlock Plus
Recommended Chrome Extensions – AdBlock v2.0