I use Symantec Client Security; it may not be the best, but it non-intrusive.
I beg to differ. Symantec is one of the best of windows but their Mac offering is horrible. You can hardly configure anything it auto scans usb drives and disabling the autoscan is quite hard. Oh and top all that it actually uses and insane amount of resources. I went from Symantec to Sophos and it's been great. Sophos has an ugly interface and is not native at all but it does work and is quite light on the system. I get it for free atm so I can't tell you if it's expensive but if I had to recommend one Sophos will be it. Oh and I've tested all the commercial offering. Some are really pretty but ultimately you need something that works great and looks have nothing to do with that.