There are different product keys for different versions of the software. Your server came with OEM software, which has an OEM product key. That product key may not work for other OEM software, and will certainly not work for a "retail" version of the software, or for Technet/MSDN software. Likewise for the trial software, with the added caveat that the trial version of the software
will expire and there is no way you can prevent that no matter what product key you use.
Get yourself a software package and product key that match. It's the only thing you can do. For more information about Windows Server 2008, see the
appropriate forums .
I'm not on the WHS team, I just post a lot. :)