arnox
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11/26/08: I haven't tried every virtualization product out there, but VirtualBox scores high on simple installation and ease of use. The software runs on all popular host operating systems, such as Windows, OSX, Linux (most popular distributions) and Solaris.
I run WindowsXP in VirtualBox on Ubuntu 8, and have a few friends that do it the other way around (Ubuntu on WindowsXP).
If you have the need for virtualization, take VirtualBox for a spin.