Many documentations and forums note that Exchange 2007 SP1 cannot be installed on Windows 2008 R2 Standard/Enterprise. This is not supported, but it can be installed for some people. For a few others, there is a modification that is required when the Mailbox Role fails.
Found on MS Technet Social Forum posted by Aahan:
Open C:\Program Files\Microsoft\Exchange Server\Setup\Data\admin.ins and delete everything under
--Run the installation again and it'll be in maintenance mode and the only thing left to install (besides the UM) is the Mailbox Role. Run the installation and everything should complete correctly at that point.
When doing the original installation, some IT administrators opt to run the Setup in Compatibility Mode, but this is not required. Just make sure to run it in Administrator mode. As well, turn off / disable UAC on the server. Else, you will have a lot of access denied errors.