When you try to install Exchange Server 2007 SP2 on a Windows Small Business Server 2008, installation process doesn’t start, and you receive error message: “You must update your Windows Small Business Server 2008 settings both before and after you install Exchange Server 2007 SP2, read detailed information at http://go.microsoft.com/fwlink/?LinkId=155135.” This problem occurs because Exchange Best Practice Analyzer prevents Exchange 2007 SP2 from being installed on the server. To resolve, manually install Exchange 2007 SP2 using above link, or use Exchange 2007 SP2 Installation Tool for Windows Small Business Server 2008.
974271 Description of Microsoft Exchange Server 2007 Service Pack 2 Installation Tool for Windows Small Business Server 2008. Note: You’ll first need to update Windows Installer to version 4.5 before you can install this tool.
942288 Windows Installer 4.5 is available