I heard about this software on a show called 'Screensavers' on TechTV ... which is an awesome program if you are lucky enough to get the TechTV network.

I use this program on a number of computers running everything from Win 98 to XP (sorry, no Mac )

It's free and has free automatic updates to the virus definition files.

I have found it to be very stable very easy on system resources.

AVG Antivrus System (http://www.grisoft.com) can be downloaded from the company's website (click link) at http://www.grisoft.com