Activated Windows Server 2003 asking to re-activate.

I am having Windows 2003 server on my network. I had installed the new edition 2005 and activated it. It was a proper upgrade. But this morning when I started the system the activation wizard popup again and it ask for re-activating the OS. I had checked in error log and found a error related to Windows Product Activation. I am doubtful about this problem. I think once Windows is activated we do not require to do that again. Is there some bug with upgrades.

The activation failed. You can re-do that. If you cannot find option through online mode then you can try the activation through phone. There can be some issue with internal dll file which is causing the issue. Type the following in run box one by one - regsvr32 regwizc.dll & regsvr32 licdll.dll. And hit ok, then see whether the problem is solved or not.