Microsoft has never really acknowledged or supported those among us who choose to build their own PCs. Windows licensing is usually offered in three forms: full retail product license, retail upgrade license, and OEM license. If you want to build your own machine at the moment, Microsoft expects you to buy a full retail copy of Windows, and states clearly that using the OEM licensed version is prohibited by individuals. The price difference made the OEM version the one to get, though.
With Windows 8 that all changes