From LedHed's Wiki
If you get the following error when trying to activate Windows 2008 R2 against a Key Management Server (KMS) try the following:
Error: 0x8007000D
http://support.microsoft.com/kb/2230957
If that doesn't work try running these commands:
slmgr.vbs /rilc slmgr.vbs /rearm
Restart
slmgr.vbs /ato
Error: 0xC004F038
Typically you see this error pop up on client computers when you've setup your first KMS server, or migrated to a new KMS server. A KMS server won't process activations until it gets 25+ unique client activation requests.
So you will see this error for a while. Once you hit the 25 mark clients will activate normally.
You can check the current count by running this on the KMS Host:
cscript slmgr.vbs /dli | findstr "Current count:"