OneCare comes last in every independent test on catching nasties that want to screw with your computer.
But at least it's free you say ?
Actually, a) it's not free forever, and b) It will delete important files on your computer.
It has been caught red handed deleting the entire Outlook mail file. Given that not everyone here is a techie, let me explain that unlike every other anti virus programme in the world, it doesn't just delete mails that contain viruses, it deletes every mail you have ever received.
Yes, really.
MS response for this is that these experiences make it a better product. As someone who's done computers for more than 30 years I will share with you thar this is incompetence on a scale rarely seen, not a "new lesson to be learned"
Some of the full horror is here