ComputerWorld has some details about the built-in antivirus solution of Windows 8. The site spoke to McAfee and heard that the integrated Windows Defender is basically a last resort, it will be active only on PCs without antivirus or after other products have expired.
According to Gary Davis, director of global consumer product marketing for McAfee, Microsoft has given third-party vendors and computer makers first crack at protecting PCs.
Computer makers, known as OEMs (original equipment manufacturers), typically include trial versions of major antivirus programs with their PCs. Those trials offer malware signature updates for limited periods, sometimes as short as 30 days. When the trial expires, PC owners are encouraged to upgrade to the full paying edition, which usually offer signatures for a year at prices ranging from $40 to $90.
Windows 8 cedes the AV role to those pre-installed trials, said Davis, and will not activate Windows Defender if it detects an active antivirus program that's receiving signature updates.