Two free pieces of software that many pros rate very, very high...
PC Tools Threatfire: Real time protection against day-one infections. Doesn't compare files to a database, but looks at your computer's activity and stops malware dead in its tracks. Cutting edge technology. Rated 5 stars.
PC Tools antivirus: One of the top 5 rated.
Both are absolutely free and are updated daily, within 5 minutes of starting your machine.
One suggestion though, shut down the antivirus before playing AH2. Right click on the icon in the system tray and click "Shut Down". Restart later using the desktop shortcut. You can leave Threatfire running while playing, it doesn't scan packets and uses little resources. Threatfire will block know bad sites and prevent known threats from infecting your machine as well as stopping day-one threats (the kind that don't show up in antivirus updates for a week or two).
Many will have their favorites, but PC Tools offers the best protection in terms of free software.
My regards,
Widewing