Or virtual anything else: I've been playing around with the open source edition of VirtualBox, a utility from Oracle (boo!) that lets you run another operating system in a Window in your current operating system.
Any host operating system: Linux, Windows, OS X. Any target OS: Linux, Windows, OS X — well, maybe you'd better ask Steve about that. Tell him I said it was OK.
The open source version of VirtualBox is included in the Ubuntu repositories, so install it with:
sudo apt-get install virtualbox-ose
On your Gnome menu you'll find and entry for VirtualBox OSE under Applications => Accessories
Click New and follow the directions. I was able to install Windows XP without the usual hassles. VB provided all the needed drivers.
OK, one problem. The virtual machine wouldn't boot. The problem turned out to be that VB was looking for a floppy disk drive, which Hal doesn't have. I had to click Settings => System, and then unclick the Floppy Drive label under Boot Order. After that, everything worked fine.
I was even able to stream Netflix movies in a browser under Windows, once I installed Silverlight.
Useful for the very few times I need Windows.