Installing Exchange 2007 Service pack 2 on a Windows 2008 SBS generates an error, as detailed here:
To resolve, you can either follow the instructions outlined in the KB, or you can download the installation tool, especially designed for SBS 2008.
The tool prepares the SBS 2008 for the installation of SP2 by turning off various auxiliary services (including Forefront Security), installing the service pack, then turning them back on. When not using the tool, the installation of SP2 will fail because of safety tripwires in the Exchange SP2 installer. A more detailed explanation is outlined in the KB: http://support.microsoft.com/kb/973862.