I recently bought Windows 7 OEM system Builder Pack, Home Premium for 89 bucks on Amazon and got it today (sunday, weird right?) and i was reading the back and it said "If the individual software liscense is for a desktop operating system or application software, it also must be pre installed on the hard drive of the fully assembled computer system, using the oem previnstallation kit (OPK)
OPK is for major and local PC stores that need to have Windows pre-installed on the fully built computer before the customer purchases it.
Basically it's like when you buy a computer from like Best Buy, Windows 7 comes already installed but not activated until you take it home and do it yourself. They do this using the OPK, as they are not allowed to sell a previously activated version of Windows.
As far as I know, OEM versions of Windows get tied to the computer they are installed on. If you are ever planning on changing the motherboard or CPU, then you may have to purchase a new copy of Windows 7.