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.