There was another time where I had activated Windows server 2008, and I honestly did do it, BUT 7 months after installing it fell flat on my face because when I logged back on to the server it showed me that it was not activated, even though I did activate. After pulling my hair out, I found out that the Symantec Endpoint Client was the culprit. Symantec sucks.
I removed the Endpoint Client, rebooted and VOILA!! The server said it was activated. In the end, a crap load of Server updates were run, and THEN I added the Endpoint Client back and now all appears to be heavenly.