Windows XP Mode was a freely activated copy of Windows XP from 2001 that ran on Windows 7 Pro and Win 7 Ultimate in the old Connectix Virtual PC. Microsoft acquired Connectix Virtual PC and made it into a a standalone installation package which had to be installed separately from an installer that contained a Windows VHD image of a Windows XP machine called Windows XP Mode.
Windows XP Mode did not need to be activated online, since it relied upon a new version of Activation called OEM BIOS Locking.
The Microsoft Virtual PC software has a form of virtual BIOS which contains the activation key for Windows XP Mode, so after installation was complete within the Virtual PC instance it would autoactivate, or already be activated.
VirtualBox is a slightly more advanced virtual machine kit which comes from Oracle, but also runs on many Windows, Linux and MacOSX and MacOS platforms.
VirtualBox also has a way of loading a BIOS image if dumped from a running systems memory using a tool like SLIC Toolkit. By modifying the dumped image and adjusting the Virtual Machine Settings using the VBoxmanage.exe tool to load it.. the Windows XP Mode image can be loaded into Virtualbox on not only Windows, but Linux, MacOSX and MacOS..
The Windows XP Mode VHD must be converted first however using the VBoxmanage tool to clonehd the image and export it as format VDI.
The placement of the BIOS activation string within the modified BIOS dump also moves around slightly from VBox 4.0 to 5.1-5.22 to 6.0 by a single or more bytes. The BIOS is compiled differently for each version of VBox. This technique is known to work on those versions, but the latest version is 7.0 and it is not known if it can still be made to work on that version.
VirtualBox is also know for supporting USB pass thru and offers a more robust long term way of supporting XP SP3 as activated on more varied and modern os platforms in the current year.
