Microsoft Security Essentials
Microsoft recently released the full version of it's free Security Essentials anti-virus program, available for immediate download to anyone with a genuine Windows XP/Vista/7 system. The tool "guards against viruses, spyware, and other malicious software" both in real-time as you download/open new files and via scheduled static scans. It can be configured to do one or the other or both depending on your needs.
Initial reviews on both virus detection rate and system resource usage for Security Essentials seem positive. I typically run my Windows 7 system with UAC turned off (don't get me started), Windows Firewall on and no virus scanner. With some safe browsing practices and not running crack.exe programs under any circumstances, I've managed to stay virus free for a long time now.
I've used third-party virus scanners in the past, but my experiences have been pretty bad. Over time, all virus scanners seem to bring the system to a crawl. Older versions of Norton were especially bad at this. My previous top-of-the-line laptop used to take over 10 minutes to restart due to it.
I'm willing to give Security Essentials a chance and see how it goes. I'll post an update after a few weeks in regards to performance. Hopefully it won't cause any noticeable degradation...