With the release of Windows R7 from Microsoft as a free beta, Mac users can now install Windows for free, and by all accounts it runs faster than Vista!
In theory, it should be possible to run it in any virtual environment, such as Boot Camp, Parallels or VMWare Fusion. However, another option is Sun’s VirtualBox application which is free.
VirtualBox is a family of powerful x86 virtualization products for enterprise as well as home use. Not only is VirtualBox an extremely feature rich, high performance product for enterprise customers, it is also the only professional solution that is freely available as Open Source Software under the terms of the GNU General Public License (GPL). See “About VirtualBox” for an introduction.
Presently, VirtualBox runs on Windows, Linux, Macintosh and OpenSolaris hosts and supports a large number of guest operating systems including but not limited to Windows (NT 4.0, 2000, XP, Server 2003, Vista), DOS/Windows 3.x, Linux (2.4 and 2.6), Solaris and OpenSolaris, and OpenBSD.
VirtualBox is being actively developed with frequent releases and has an ever growing list of features, supported guest operating systems and platforms it runs on. VirtualBox is a community effort backed by a dedicated company: everyone is encouraged to contribute while Sun ensures the product always meets professional quality criteria.
You can follow the VirtualBox blog here.