If your laptop is still running and bootable into windows you can use http://www.magicaljellybean.com/keyfinder/ to get your COA key. Now if your laptop is an OEM load from HP Dell or Gateway or other large manunfacturer then they may have used a OEM Volume License Key and it will be a different number than what was on your tag.
One way you can tell this is if you recover your laptop from scratch, and you don't have to enter your COA key, then the recovery media was pre encoded with the OEM volume license key.
Dave