I don't think you can really compare these two. Not in a fair manner anyway because there are so many other things going on 'under the hood'. A regular FreeBSD box for example has several processes running in the background, DBus and Hald being the most obvious, but the same can be said about Windows. It wouldn't even surprise me if Windows had more and more intense processes running than FreeBSD.
For what's it worth I'm under the impression that KDE4 and Windows 7 are pretty much on-par where responsiveness is concerned. But the fun fact is that I'm also comparing apples and oranges. My Windows 7 is a 32bit machine whereas my FreeBSD box is a 64bit server which also happens to run KDE from time to time (I'm usually not logged in).