I don't go to any websites using my totally legal and legitimate copy of Windows. This VM is to run one program (ArcGIS), and it is turned off when I'm not using it. And it does okay. I have 2GB RAM, and when the VM is on, it uses 1GB, leaving 1GB for OSX. Fortunately, the way I've got it set up, there isn't any desktop overhead, so when I'm running ArcGIS, it gets most all of the memory - which is good, because it's a real hog. Everything is a little bit slower, obviously, but not unbearably so. The real trouble is overheating, which happens a lot with this machine, even before installing the VM. If I need to install software, or render something, I kick the up the rpms on the fans.
I installed AVG-free today. Mostly to keep myself from becoming a transmitter. I'm pretty cautious with Windows, and totally carefree with the Mac. I'll probably see if I can go without Windows updates for awhile. But eventually, I'm going to have to get some of them, because there's an extension I use which requires a dotNet upgrade, which requires a bunch of other updates.