I am not sure what you want here. VNC programs are the best at cross platform control of computers. For windows, you can install a VNC server as a service and it is in the background, no visable sign of the VNC server. Try UltraVNC for windows. I am sure there is a Mac VNC server that would work much the same. Yes, the client is in a window, but that is a good thing, you can move it where ever. Or, if you are like me, and have two monitors, you can run your Mac system in one screen and your windows in the other. Very handy.

First, I don't care for RealVNC, Ultra preforms much better, in my experience. Second, that is anything over a network for you. There is a natural latency over a network, it is not seen durning normal tasks like IMing or playing games, but it is there, just not very noticable. With VNC, a rather large amount of data is being sent in a very short amount of time. But like I said, you are not going to find anything better that is cross platform.

might want to check out TightVNC too- i've had good success over the internet with it. it seems to have a little better compression. of course with software like RealVNC and TightVNC, you can adjust things to improve your experience.