Unfortunately, when you buy a computer with a preloaded OS, that OS is bound to that computer. Legally if you sell the computer you are suppose to include those restore CDs with it. You also may not transfer one copy of the OS to another. If you go out and buy a plain retail box or even OEM windows disc by itself then you may install it on any machine you want, but can only have it installed on one machine.
Its all in the EULA, and I used to work for a MS reseller back in the day and I had to do OEM installs of the OS and Office. If clients returned machines we were not allowed to remove office, we had to resell it with that version of office preinstalled on it. Same thing went with windows.
Of course there is no real good way to track that info down, which is why MS went to having activation codes.