For the most part - what Teaser said
Most of the viruses written today are written for loopholes in Microsoft software that have been addressed but people have not patched. So that is where being dilligent with patches is good.
Most problems today are technically worms and not viruses. The worms usually require some active invovment from the user - like clicking on the attachment - to activate.
As for Norton -v- McAfee -v- Trend -v- .... They are all fairly good but you can still get bitten by something that comes thru before they can make the updates. Yes, there is a scanner that checks for malicious "like" properties but...