Install of client access role fails on Windows Server 2008 R2

Trying to install Exchange Server 2010 onto Windows Server 2008 R2 which has also been setup as a domain controller. When attempting to install the Client Access Role, setup fails with the error below.

Does anyone know to a way to get around this?? Please shed any lights I would be appricated.

Thanks,

Tom

Error:
The execution of: "$error.Clear(); Install-ExchangeCertificate -services "IIS, POP, IMAP" -DomainController $RoleDomainController", generated the following error: "Could not grant Network Service access to the certificate with thumbprint 2F320F5D5B5C6873E54C8AB57F604D8AFA31D18C because a cryptographic exception was thrown.".

Could not grant Network Service access to the certificate with thumbprint 2F320F5D5B5C6873E54C8AB57F604D8AFA31D18C because a cryptographic exception was thrown.

Access is denied.

Elapsed Time: 00:00:01

Mailbox Role
Cancelled

Finalizing Setup
Cancelled