PC World writes that Microsoft has started alpha-testing their Antivirus program. The program is said to be working as a 'behavior blocker' which monitors different events and actions on computers. If the event or action is typical of virus or harmful, it will be prevented.
Behavior blockers do not use code signatures like traditional scanner-based antivirus programs, so they may be able to protect against new types of viruses without being updated--an important consideration for Microsoft.
The antivirus product was also referred to as an Intrusion Detection and Protection System by sources, indicating that it may work in conjunction with the Windows Firewall.