The problem is OEM system builders (gateway, HP, Dell...etc), often use OEM windows versions. They are cheaper to buy, so the cost of their systems are cheaper (so they say....).
The problem comes, when a customer wants to do exactly what you're trying to do. When an OEM version of windows is activated, that activation is tied to the original hardware (namely the motherboard). If you put it on a new motherboard and try to activate it, it won't work.
Even if you borrow a CD from a friend, your activation key still won't work on a new build. Unfortunate that it is, this is the downside of OEM systems that most customers don't know about.