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.
Its Sun Javas answer to Microsofts Virtual Machine (or VM). And allows you to section a portion of your HD (not partition) to run a secondary Operating system from within the Windows operatings system.
It make this one really big file on the HD. Personally, I would try the Sun Java one and see if it will give better functionality to the hardware as the Microsoft one is limiting and set my nVidia chipset up as an Intel costing me some functions.