With Windows 8, Microsoft acknowledges for the first time installations on home-built computers.
ZDNet got a hold of the new license agreements that affect the upgrade sold via retail, the OEM license agreement as well as system builder license relating to personal use systems. If you are building your own PC, you can, effectively take advantage of OEM versions instead of having to purchase a full and more expensive retail license.
ZDNet quotes the personal use license for system builders and Windows 8 Pro as follows:
We do not sell our software or your copy of it – we only license it. Under our license, we grant you the right to install and run that one copy on one computer (the licensed computer) as the operating system on a computer that you build for your personal use, or as an additional operating system running on a local virtual machine or a separate partition, subject to the restrictions outlined under “Are there things I’m not allowed to do with the software?”
Note that Microsoft also takes virtual machines and multiple operating systems on the same computer in consideration. This is also a first for Windows. According to ZDNet, Microsoft currently sells Windows in three packages - retail upgrades, full retail licenses , and OEM System Builder packages that are not permitted to be run on personally built systems. This appears to be changing now with Windows 8.