I've been using
VirtualBox to run a virtual Windows XP PC on my Ubuntu Linux system. I have always found virtualization software like
Qemu or
VMWare to be a bit fiddly to install or (once you get the thing working) slower than a peg-legged tortoise doing Tai-chi.
VirtualBox is free, open source (there is a commercial version with a few extra features) and will let you run a variety of guest operating systems on top of Windows, Linux and Mac OSX. It is also
fast. Even on my rubbish home Athlon it still runs fast enough to browse the web, run MS access and play a few (older) games.
Desktop integration is very well handled and you can either run the entire desktop in a window on the host PC or let programs run as if they were native apps in their own windows.