nt evaulation license?

We are getting an error from an NT box saying that the evaluation period has expired and the system will shut down in one hour. We’ve double checked the license and it is the original one that came with the box. (I thought perhaps I had accidentally used the number for a training version I had used on a pc for some classes that had a 90 day limit, but the license is the correct full version). Is there somewhere in the registry that turns this on and off? We’re hoping not to have to reinstall NT if possible. Thanks for the help