You might find that a COA license for the laptop will not be accepted if you use media that is not coded for that license (Certificate Of Authenticity HAS to be stuck to chassis under microsoft licensing rules [if the OEM is using the COA scheme], this might be the serial number you find). COAs are a 'cheaper' license available to some OEMs. The OEM is responsible for ensuring that you can get the install media. This used to mean they included a coded disk with the original machine. This had the advantage that it included a slipstreamed copy of enough drivers to get all of the hardware working (thousands of updates available for download probably though). With the internet becomming more prevalent, and in an attempt to cut costs, a lot of the OEMs now provide these disks from their websites. If it is a dell or hp, put the service tag/serial into the support site, and it should take you to a page to download the disk fron (read the page carefully, they like to hide them for some strange reason)