Microsoft licensing is a bit of a minefield. I lot of people are shifting second hand laptops on ebay without Windows install CDs and using the fact that they have a COA stuck to the unit (and there seems to be someone selling such stickers on ebay at the moment) as some sort of suggestion that they're properly license.
I do know that some OEM installations become tied to the machine they're installed on and presumably could therefore be sold on without the CD but in general I'm not quite sure where I stand.
