IIRC, Tiffany is half right. OEM versions put a SLIC, or something to that affect, into the motherboard. For example, you buy a HP computer, so when you install the Win7 OEM you got with the computer, if its not being installed on the HP motherboard it won't let you finish the install. I don't believe that full retail copies do this only because they're built for multiple installations on different computers that have different motherboards, just need to deactivate, or somehow, turn off that brodcast signal. Not sure if the brodcast signal is the same coming off the motherboard.
I could easily be wrong as well and would love indepth explanation (or I'll probably just go look it up myself).