Starting with Windows Vista, Volume License Keys (VLKs) have been replaced by the Key Management Service (KMS). A KMS Server is deployed in the organization, which will respond to activation requests from client machines. Hosts activated via a KMS have to report back to that KMS key server once every 180 days.
At the place where I work, a Key Management Server was already in place and it was used for activating Windows Vista. With release of Windows Server 2008 earlier this year, we started deploying/migrating some of the workloads to the new server OS.
Windows Server 2008 also uses the KMS activation system. However, the KMS server refused to activate Windows Server 2008 computers, while Vista was OK. Upon closer examination and long discussions with Microsoft, it was discovered that the KMS key installed on our KMS server was only for Windows Vista. Microsoft calls this a "Class A" key.
In order for the KMS server to be able to activate BOTH Windows Server 2008 AND Windows Vista, we should remove the "Class A" key, replace it with a "Class B" key and then activate the KMS server. Here, the "Class B" key is nothing but the Windows Server 2008 Std/Ent – KMS key on the MVLS website. This key can not only activate WS08, it can also activate Vista!
This is how its done (from the Command Prompt):
uninstalls our "Class A" KMS key.
slmgr -ipk <insert Windows 2008 KMS key here>
installs the "Class B" KMS key.
activates the KMS server
While using slmgr, it is important to wait 5-10 seconds after each command to get the confirmation popup box, even though command prompt will return as though nothing happened.
Once the server has successfully activated, type the following command to verify that you have a "Class B" KMS key.