Error Installing Exchange 2010

Hi

I'm following Dematzer's guide on Migrating from SBS2003 to Windows 2008 and Exchange 2010.

All steps completed thus far with no issues.

I am on the Exchange installation part, and the readiness checks all pass, with the exception of Organisation Prereq, but according to the guide this is normal as we are allowing Exchange to do the /PrepareAD bit.

As soon as I hit Install it fails on the first step Organisation Preparation:

Summary: 12 item(s). 0 succeeded, 1 failed.
Elapsed time: 00:00:00


Organization Preparation
Failed

Error:
The following error was generated when "$error.Clear(); buildToBuildUpgrade-ExsetDataAtom -AtomName OrgLevelCt -DomainController $RoleDomainController" was run: "An error occurred with error code '3221684229' and message 'Access is denied.'.".

An error occurred with error code '3221684229' and message 'Access is denied.'.

Elapsed Time: 00:00:00


Preparing Setup
Cancelled




Stopping Services
Cancelled




Copy Exchange Files
Cancelled




Language Files
Cancelled




Restoring services
Cancelled




Languages
Cancelled




Management Tools
Cancelled




Hub Transport Role
Cancelled




Client Access Role
Cancelled




Mailbox Role
Cancelled




Finalizing Setup
Cancelled

All google brings up is stuff about Exchange 2007...

Help!