MacOS and some versions of Linux have built in anti virus services that are maintained by the organisation that made the OS. And looking at the market I wonder why Microsoft didn't act sooner, but they are now brining out OneCare which is designed as a comprehensive system tool for defending and maintaining a system.
I think this is a great move because it is specifically targeted at home users that are in the high risk category for owning a computer that is infected with multiple virus, and offer then a great hands free solution. This might even go further and force McAfee and Symantec to cleanup their act because they have always dominated this market will rubbish excuses for security products and have probably contributed to the problem more than providing a solution.
OneCare will be a subscription service, but if the AntiSpyware beta is anything to go by this will be an excellent product.
I will probably stick with NOD32 because of the extra control it gives me, but I look forward to seeing people use OneCare over Norton.