In addition to what ThatVietGuy states, if the copy of Windows 7 you have is an OEM version, then, per the license agreement, that product key is tied to the first motherboard you install it on and it cannot be installed onto any other system.
I would note, however, that many people have reported success at installing and activating an OEM version of Windows 7 onto another system. You may need to contact Microsoft over the phone in order to have it activated successfully.
-Wolf sends