I've been using VirtualBox w/Win XP on VL for a while now. I don't fire it up that often, since I usually don't need Windows except when someone calls me with a Windows-specific question. Having said that, Windows on VirtualBox runs pretty well; I imagine it's faster or slower depending on your processor and how much RAM and video RAM you allocate to it (you can only allocate 32 mb of video RAM). If you want to share folders between Windows and Linux, access USB devices and (I think) have full screen mode you'll need the full version(which has the guest additions), which is "free as in beer" but not Open Source, and which can be found here: http://www.virtualbox.org/wiki/Downloads
. This version is free for personal use.