As already pointed out there is no key in the CD.
When your system was installed it will have been installed with a particular key.
There are different types of key. The main ones being VLK, OEM and Retail. There are two types of OEM which I term as "SLP OEM" and "Generic (or boxed) OEM". "SLP OEM" only come with pre-built PCs from major OEM suppliers such as Dell and HP. "Generic (boxed) OEM CDs are generally bought together with PC hardware such as motherboards etc from an approved PC component seller
Each of these have different characteristics, are purchased differently and require different media to Install the OS.
If you have an "SLP OEM" install (e.g. Dell, HP, etc) then the key that is attached to your case will not be the same as was used in your system. These types of systems are known as SLP BIOS locked systems and the key used is a special type of Volume Licence Key for use by OEM suppliers such as HP, Dell etc. These keys can only be used in those system provided by these major OEMs and are locked to the PC BIOS.
The VLK (Volume Licence) key is used by and sold to major corporations and businesses with multiple PCs.
Both the above do not require activation with Microsoft.
The OEM key that appears on your SLP BIOS locked PCs case can sometimes be activated by a manual call to MS. You have to however, convince them that you are the legitimate owner and that it has not already been used by someone else.
The Retail key and "Generic OEM" key requires activation with Microsoft during the install phase. This is generally a simple process and involves an automatic online activation towards the end of the install (assuming the PC has the capability to connect to the internet).
There are tools available which will allow you to extract the key that is currently in use in your PC.
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☺ When everything else fails, read the instructions.