Exchange Server 2007 Install Problem

I’ve setup a new Windows 2003 64 Bit Server for running Exchange 2007. I’ve installed everything (twice now on separate drives) and Exchange Server is giving me the same error:

D:\>setup /preparead

Welcome to Microsoft Exchange Server 2007 Unattended Setup

Preparing Exchange Setup

Unhandled Exception: System.TypeLoadException: Could not load type ‘Microsoft.Ex
change.Setup.ExSetup.ExSetup’ from assembly ‘ExSetup, Version=8.0.0.0, Culture=n
eutral, PublicKeyToken=31bf3856ad364e35’.

Exchange Server setup encountered an error.

Any ideas what this means I have to fix? I have AD setup and running, and the computer is all udpated and appears to be working just fine, but this error message isn’t telling me quite what I need to fix.