Jim Hamm gets our attention with this: "Here is an interesting article from Thomas' Tech Corner describing a series of tests he ran on anti-virus programs. And here is a summary of the results in PDF format.
"Avast AV, which I use, came in a close second to first-place Sophos. The author devotes a paragraph to the problems Norton AV caused. Most (maybe all) of my computer friends don't use Norton, and immediately delete it (as I do) if it comes pre-installed on a new computer.

"Some might say an AV program is not needed on a Mac, but if I can get additional anti-virus protection free, and it doesn't cause any problems otherwise, I'm all for using one." Thanks, Jim for the info.