Windows 7 Windows XP mode cannot run - it says "Require Hardware Assisted Virtualization"

"Your machine may support it, but not have that support enabled. If your CPU supports hardware assisted virtualization, there will be a switch in the BIOS that will enable it.

What CPU does your system have? If its an AMD CPU, it will support hardware assited virtualization unless its a Sempron. If its an Intel CPU, then finding it on the following page will tell you whether or not it should support hardware assisted virtualization:

http://ark.intel.com/VTList.aspx

UPDATE:
If you have it, the BIOS option may not be immediately obvious. On an AMD based machine it may be called ""AMD-V"", or on an Intel machine it may be called ""Intel VT-x"". If you don't have an option called ""hardware assisted virtualization"" look for some variant on the above two names.

UPDATE 2: Following your comment, it looks like you won't be able to enable the virtualization support without changing your CPU :("

Turn it on in your BIOS. If you don't have it (some motherboards don't have this feature), you'll not be able to run XP Mode, but you can run software virtualised VMs like in the past.