Normally one antivirus on one computer is sufficient.

Nowadays, antivirus are not as needed as before because instead of p2p, we go torrent now. How many of us has encountered antivirus on torrent files?

I previously used Norton until came 2005, I switched AVG cause it's free. Norton changes EVERY year. Whether you get the original or the pirated, it's still TOO troublesome.

To me, basically all antiviruses are the same
Norton
McAfee
Trendmicro
AVG
Avast
Sophos
Ahnlab

They basically just scan for virus which in my opinion is quite rare these days. Even webmails like gmail, hotmail, etc scans for viruses first before you download your attachment. What more is there?

Having two antivirus will usually clash your system. It happened before in the older days but til now, I doubt there's any two antivirus that can run parallel compatibly.