One of the all-time-favorite articles here is “Running your dual boot windows inside Vmware Server within Ubuntu“. Yet some might like it the other way around, and a bit more seamless? Two handy options here are “VirtualBox” & “Ulteo Virtual Desktop“.
You can put Virtualbox in the same range as Vmware. They provide a performant virtualization platform. Yet one of their core features (which I still miss in Vmware, apart from Fusion) is the “seamless” mode. It means your guest OS can appear on your desktop without having to be inside a window. You can run application (say notepad) and it will appear as a single window on your desktop.
With Ulteo Virtual Desktop, you just have to run the application you need to use from the Ulteo panel and its window will show up like any other Windows application. Ulteo steps in to avoid the point where you need to maintain your virtualization platform (as with the seamless solution of Virtualbox). See it as an application that will (install and) maintain the seamless solutions for you. Check out their website (and the /. comments).