Microsoft's Windows 8 will activate its built-in antivirus (AV) software only if it senses that the PC is not protected by another security program, according to AV vendor McAfee.

The new operating system, which last week reached its final public milestone, includes security software, dubbed "Windows Defender," that combines characteristics of both that anti-spyware program of the same name and the free Security Essentials, the antivirus program that until now has been offered as a separate download.

Microsoft first announced Windows Defender last September, and included it with that year's Windows 8 Developer Preview, and this year's Consumer Preview and Release Preview. In all three sneak peeks, Defender was switched on, and served as the operating system's default protection against malware.

But that's not how it will work when new Windows 8 PCs debut later this year.

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.

"Looking at the way they're doing things in Windows 8, Microsoft is going to great lengths to let AV vendors keep customers," said Davis in an interview last week.

But Microsoft didn't take the tact to please security companies.

"It is our understanding that this was mostly because of pressures from the OEM community," Davis said in an email replay to follow-up questions. "A large portion of their profits on PCs come from revenue associated with AV."

Computer makers do reap revenue from the pre-installed software, including antivirus tools, that they bundle with their PCs. In the case of AV software, OEMs receive a portion of the money paid to security vendors by customers who upgrade from the trial versions to the full editions.

That practice relies on loading the PC with a range of third-party applications -- not just AV products -- that critics scorn as "crapware" or "bloatware."

Windows 8 will activate Windows Defender automatically only if there is no other antivirus software on the computer, said Davis.

And even when the OS detects that existing AV software hasn't been updated recently, it will only offer Windows Defender as one of several choices.

When the PC stops receiving AV signature updates -- most likely because the trial version has expired -- Windows 8 begins a 15-day countdown. During those 15 days, the Action Center, a desktop component that consolidates important system notifications, will warn the user that the AV software is expired, with information about how to renew coverage.

After the 15 days, the warning will expand the options offered users.

"At the end of 15 days the user has the option to renew what they have, activate Windows Defender, select another option from the Microsoft Store or click on a 'remind me later' button, which starts a seven-day notice period," said Davis.

The Microsoft Store is the name of the company's online market, where it sells its own Windows software, including operating system upgrades, as well as some third-party programs. It's not to be confused with the Windows Store, the e-mart accessible only from Windows 8 that is the sole distribution channel for Metro-style apps for that OS and Windows RT.