When you say you want the computer to keep the same name, I take it that it's a managed computer? I ask because I think I've seen that error if I mistype the GUID, so it tries to use the string %machinename% as the actual computer name...
(I've spent the last 3 weeks setting up a server 2k8 network from scratch, and have learnt a hell of a lot in that time, so forgive me if I'm a little vague on individual error messages! )