An OEM licience for windows will be on a sticker on the side of the computer, if you use the code to activate windows on a different computer, you are breaking the law.
"The licience dies with the machine", this is what i have learnt.
I suppose that's an odd question to me digger. The Win7 OEM is actually for home users to use on 1 computer only. Not 2 computers or your computer and a friend's computer. 1 computer.