Within the free WindowsXPMode_en-us.exe file, there's a VirtualXPVHD file containing the Windows XP virtual machine, which you can open in VirtualBox. Here are the steps to get this working:
- Download WindowsXPMode_en-us.exe from Microsoft. You'll need to run the validation tool to prove your copy of Windows is valid.
- Then use 7-Zip or another archive tool to open the EXE file as an archive.
- Within that archive, find the "sources/xpm" file within it, and extract that folder to your hard drive.
- Finally, in the extracted xpm file, you'll find a file called VirtualXPVHD. Rename it with a VHD extension.
In VirtualBox, open the VirtualXPVHD virtual machine, and voila! You've got Windows XP Mode (running Windows XP Professional) in Windows 8, no need for the XP license.
This might also be possible with Windows 8's built-in virtual machine manager, Hyper-V, but that's only available in Windows 8 Pro.