I have a XP Home instance for my wife, a high school teacher who from time to time needs to use proprietary, Windows-only software for things like generating report cards and such.
I also have an instance of XP on my MacBook Pro loaded with some Windows-only audio and video production software to deal with some clients who have begun a project on that software and need me to do something.
Often, too, when I am installing Ubuntu (and now Debian instead of Ubuntu. I have found Ubuntu *perfect* for first time users, but if I am to administer the system, and don't need all the hand-holding, I prefer the speed and stability of Debian) on a friend/ family/ neighbour/ client's system, I will take their original Windows installation and mirror it as a VM, both so that they have all of their data (just in case I forgot to migrate something), and to continue to be able to use whatever Windows-only stuff they need.
I use VirtualBox 3 (the non-OSE, or Open Source Edition, which gives me access to USB devices whereas the OSE does not).