I would not consider McAfee Antivirus, even for free.
It is a resource hog with six or so programs running simultaneously, including somehting called security center.
They used to be the best, but just don't cut it anymore.
If you want a good, free antivirus, download Grisoft's AVG Free edition.
AVG
Why buy anti-virus software when you can get it for free (for personal, n-f-p groups, only)
By the way, Grisoft has changed their front page. It's less readable and more difficult to find the free version. The link I listed goes to their new front page for the free stuff... http://free.grisoft.com/freeweb.php