You do know there are plenty of program that will pull the CDkey off the machine... I would be cautious if you intend to sell this computer and use the CDkey for yourself too.
How about a discount for a No OS Option? Since it is not MCE, most people have XP already. If you dont sell the License to the Key, its just as illegal to use it as for the buyer to use his or hers on a 2nd machine.