WinPatrol (free) and WinPatrol Plus have been around for years, as a host intrusion prevention utility. It's not an antivirus as such, but the program monitors what's going on in various areas of your Windows PC, and actually barks (it's kinda cute) when something may need attention. It's always been something I would hesitate to recommend to novice PC users as it can be overwhelming in the amount of information it covers. It is a very useful program though, and works well. The new versions of Win Patrol (free and "Plus"), as reviewed by PCWorld, have streamlined the alerts so you don't hear about non-essential events, which seems like a good move if done prudently. It works on Windows XP through Windows 8.
PCWorldFor example, past versions would notify you when a legitimate program was added to your Startup program list. The latest version will notify you when a higher level of activity—typically signifying some sort of malicious intent—is detected. WinPatrol also now has the ability to learn from your responses: when you tell the program that you approve of an application's actions, it won't alert you about that action again.