Do the following.
- Open the command prompt and change working directory to System32 directory
- Type in slmgr.vbs -ckms (this clears and KMS entry you may have)
- Type slmgr.vbs -upk (this removes any product key installed)
- Type slmgr.vbs -ipk xxxxx-xxxxx-xxxxx-xxxxx-xxxxx (where the x’s is the new product key you want to use )
- Type slmgr.vbs -ato (this activates the server)
after doing that, Windows Server 2008 should show up as activated.