Avast! Free Edition
The free version of Avast! is arguably the most comprehensive set of freely available security features on the market. There's a reason these guys have more than 150 million active users (at the time this review was written). The antivirus, antispyware, and heuristics engines form a security core that also includes multiple real-time shields. Along with the new features, it's got an AutoSandbox for automatically walling off suspicious programs; a full complement of shields that guard against scripts, P2P networks, instant messaging, and potentially dangerous program behavior; a silent/gaming mode; on-demand boot scanning; and a healthy output of statistics for the data nerds.
Avast's sandbox, by the way, automatically places programs in a virtualized state when it suspects them of being threats. It walls off suspicious programs, preventing them from potentially damaging your system while allowing them to run. As the program runs, the sandbox keeps track of which files are opened, created, or renamed, and what it reads and writes from the Registry. Permanent changes are virtualized, so when the process terminates itself, the system changes it made will evaporate.