I stopped using anti-virus from 1998 (year I got mcse) to 2008.
I got one virus and manually removed it.............but I did use a firewall.
The advent or root kits, though, determines that some sort of live scanner is needed when browsing.
Browsing habits determine infection events more than anything else.
Any time I've seen a pop up window concerning "virus check" or any fake ass'd virus message designed to spoof a anti virus software message, I immediately alt/f4 all open windows rather than try to click it closed.
If you think you've experienced an "event", best to alt/f4 any application you are running, check under hkeylocalmachine/software/microsoft/windows/current version/run for something you don't remember being there, check you 'start up" on the start menu, and go through the other places an executable can be run from msconfig or compmgmt.msc..........as well....check what processes are running.
I also use DLLshow to see hidden processes and google search them to see threat level.
Once that is done, run a malware/malicious software/virus scan.
This should all be done before a restart (if possible).
I currently run microsoft security essentials and have the firewall tightly controlled and a shortcut to the microsoft malicious software removal tool but I have them set up so as not running by default and only use them while browsing and not during gaming.
I also run all security scans right before I do any online bill paying or purchasing.