W8.1 user here. I just use the anti-virus that comes with W8 (Windows defender).
When using my DAW, I just turn off (disable) my WiFi, turn off anti virus and firewall.
If I need to update any music software Apps (rarely, and I wait to I need a bunch of stuff), I just turn it all on again ([u]less than a minute to do[/u]).
I have set windows updates to let me know when windows needs updating, and let me choose when and what to download (again this only happens when I am on-line).
So when off-line, not much running in the background, other than essential windows stuff.
I have read folks on here saying there is no need to turn it all off with Quadcore i7's.
But I have VSL VE5 Pro running as a server, and it certainly all interfered with that.