I have only ran into one issue, ever, since practicing safe computer habits
.
I travel a bit, and bring my laptop. Because of this, I put a copy of Symantec Corp (Free thru work) on my Lappy (My home PC doesnt have any AV). When hooking up to a open network at a hotel my Symantec immediately started going haywire. I guess there was a PC(s) that were infected that were bombarding any other PC on the network, similar to that Blaster Virus of ~7 years ago with XP...I dont remember the Virus name, but I didnt get it :)
Thats the one and only time I was glad to have Antivirus, and also the exact reason its only on my laptop. But, if its good enough for the IT guys at my work, its good enough for me.