Originally, requirements for XP mode restricted users to specific processors from AMD and Intel that supported virtualization. Although many processors could technically handle the task, the entire situation was overcomplicated when Microsoft failed to produce drivers for all but a select few. Certain programs are able to tell you if your current processor supports virtualization, such as Securable from Gibson Research. In order to enable virtualization on the supported chips, an option must be selected in the computer’s BIOS, as it is considered an “optional feature”. Although the requirement has been removed by Microsoft, it has been stated that “XP Mode” will still maintain an advantage in terms of performance with the technology available.
The drop of this restriction has allowed for a large number of previously ineligible computers to run the software, and will inevitably result in much broader applications.
See more on Microsoft Blog: Link