My take on MS security product...

Windows Defender makes a good second layer defense as is Live OneCare. Microsoft doesn't seem to have a good reputation with Security. If they can fail an AV test meant for their product, I fail to see a reason why people should rely on it.
My 2 cents is don't solely rely on it. Many other products that have good historical/statistical results that you can DL (some for free even).. I have it but i'm not giving up my other anti-threats program as yet.

Sufficient for starters. Mind you there are plenty of firewalls which is more than basic firewall. If you happen to use ZA, you will see the increased functionality of a firewall rather than just what is seen on Windows firewall.
Gives the additional peace of mind....