XP Mode was introduced in Windows 7 as a way to help businesses migrate without losing the ability to run some of their applications. A pre-made XP virtual machine was created by Microsoft and could be installed with Virtual PC to run it on Windows 7 Professional, Enterprise, and Ultimate. Unfortunately, XP Mode required that the PCs CPU supported hardware virtualization. Microsoft recently decided to drop this requirement and issued a new patch that is installed after XP Mode and Virtual PC are installed on a computer. Go to the Download Windows XP Mode page for more information. (I had some trouble seeing the download buttons under FireFox 3.5 because the page said I was missing plug-ins, but IE worked just fine of course).