One can actually be rather surprised how great performance impact (application launch and/or boot-up times) anti-virus program do have a tendency to inflict. Not talking about 1% or 5% here... but more like 50% to 200%.
I concur that NOD32 is excellent (the only one I have found to be good for the 64-bit Vista), especially if you configure it to scan dangerous executable files only (it's perhaps only because of my usage patterns that "scan everything" do give noticeable increased latencies, even more so for network file access)!
ps. Running Linux without an anti-virus is in practice like running Windows without an anti-virus. As long as you have a firewall blocking most incomming things and you are not running any type of web/ftp/mail/you-name-it server program, it's highly unlikely your computer will ever be affected, even if you are using Windows. Unless, of course, the fault is simply "human error".