VirtualBox is a free application made by SUN. Once it is installed on an existing host operating system; within this application, additional operating systems, can be loaded and run, each with its own virtual environment.
VirtualBox is a cross platform application. Essentially, it is like having a complete and separate computer within your computer. You can install any operating system on it, including Windows 7.
The latest version of VirtualBox has support for Windows 7, which makes it very easy and convenient to install.
VirtualBox can be installed to : Windows, Linux, Mac OS X (Intel), Solaris, FreeBSD
and can emulate these operating systems : DOS, Windows, Linux, OS/2, FreeBSD, Solaris, Haiku, Syllable
Starting with Version 2.0, VirtualBox also supports 64-bit guest operating systems and starting with version 2.1, you can even run 64-bit guests on a 32-bit host operating system, so long as you have sufficient hardware.
Take a look at the video tutorial below for more information abbout installing and using this great freeware tool.