It does give OK protection, but has been known to fowl the system up when uninstalling. I've also heard reports of it slowing the system down.
However, the utilities I link to give better protection (in fact, in independant test, Avast Antivirus has been known to beat most of the commercial virus scanners on accuracy of detection of viruses).
They also use fewer system resources, so the system runs faster. Avast and Comodo use a little of the system's processing power, as does Adware when it is scanning.
However, Adware uses none when it's not scanning. Spybot also uses few resources unless it is scanning, but due to the way it works (it actually sets up the browser to prevent downloads of known badware), it can still provide good protection.
Finally, if you are using Windows XP, I would recommend you install Windows Defender from
http://www.microsoft.com/downloads/d...displaylang=en .
That last one is built into Vista.