You said Windows 2008 R2? Well, it is not supported to install Exchange 2007 SP1/SP2 on Windows 2008 R2 but Exchange can run with Windows 2008 R2 DCs/GCs. Refer below post at MSExchangeTeam blog...
Exchange Server 2007 SP2 and Windows Server 2008 R2
Amit Tank | MVP – Exchange Server | MCITP: EMA | MCSA: M | http://ExchangeShare.WordPress.com