Note the manufacturer and model as well as the serial number. Then look up the manufacturer's website and generally somewhere in their support area they probably have an OS/Recovery CD that you can purchase for about 20 to 25 bucks.
Alternatively since you DO have the Key and a valid CoA, you COULD hop onto Bit Torrents and attempt to find the OEM installation ISO. In the case of Dells this is generally pretty easy to do but if you don't get the right image, more than likely that Key from your CoA will be useless.
If you will post your model/maker and Serial Number I'd be happy to try and help you look it up from either source (the Manufacture OS disk or the Bit Torrent)