Had to set up a new PC. The first piece of software I installed was the MS anti-syware. And I really enjoyed it. It is very informative in popping up messages on what is going on. Gives you a far better understanding what all the software you are installing is doing with your machine. The good thing is that it never gets in the way, the messages pop up (in the system tray) and go away without the need to click them. Except when there is a (potential) problem. Then it will give a clear suggestion what to do.
I'm not quite sure how safe the machine is, but I do trust MS-ASW. Its usability really shines. Especially when compared to something like ad-aware.
Peter