Qemu is available for Debian's unstable distribution, and can be installed from the source code available on its homepage for Woody. It is comparible to the commercial software VMWare albeit with a few features missing and a lower performance.

On the plus side it is evolving fast, and doesnt require complicated setup or kernel patching. It is also free - although if you wish to run an installation of Windows you will need a valid license to do so.

Qemu is a complete CPU and peripheral emulator which can be used to run entire operating systems as a user process, supported operating systems include Linux distributions such as Debian, RedHat, SuSE, varieties of Microsoft Windows such as Windows 98, Windows 2000, and Windows XP and BSD based operating systems

Debian-Administration.org /.