Anybody can buy an OEM copy of Windows ...
Newegg.com - Microsoft Windows XP Media Center 2005 SP2B for System Builders - Operating Systems
This means the same as if it came from an OEM ...
It cannot be moved to another computer once installed.
The W7 "Upgrades" will be the same .. Once installed they cannot be moved ..
but forever belong to the computer it was originally installed in.
Notice that these are listed as Full or Upgrade versions.
Newegg.com - windows7