If for some good reason you can’t change the product key of your Server 2008 under My Computer Windows Activation option:
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,
Office Pro 2007, Windows Server 2008 should show up as activated.