Just been trying to install Exchange 2007 SP1 on a freshly installed Windows 2008 R2 and come up against the following error during the installation of the Mailbox Role:

Mailbox Role
Failed

Error:
An error occurred. The error code was 3221684229. The message was Access is denied..

Simple fix for this is to run setup.exe in compatibilty mode. I chose Vista SP2 and then the installation went through normally.

But before you go rushing to finish your installation it is worth noting that Exchange 2007 SP2 will not be supported on Windows 2008 R2, and therefore you may want to reconsider doing the installation at all! Read here:

http://msexchangeteam.com/archive/2009/09/21/452567.aspx

I flattened the installation and went back to Windows 2008 Standard.

Update: Microsoft have changed the policy to support Exchange 2007 on Windows 2008 r2:

http://msexchangeteam.com/archive/2009/11/04/453026.aspx