Windows 7 XP Mode
Like all good things in life - there is a "Gotcha"
Windows XP Mode will only run on Windows 7 Professional and Windows 7 Ultimate editions and requires a virtualization engine, such as Windows Virtual PC. Windows Virtual PC provides the capability to run multiple Windows environments such as Windows XP Mode from your Windows 7 desktop.
Now the Gotcha!
Windows Virtual PC requires a CPU with the Intel® Virtualization Technology or AMD-V™ feature turned on. This feature must be enabled in the system BIOS.
So if your PC is more than a couple of years old it is unlikely to be used as a virtual PC for XP, unless you are very lucky!!
Good luck.