There is an issue with installing SP2 for Exchange 2007 on SBS 2008, Microsoft has released a tool to get passed the errors - link. You need to download and extract Exchange 2007 SP2 - link, run the install tool. It will ask for the directory of SP2, and then you install SP2 normally.

Here are more details of the issue on MSExchange.org

There are additional steps to get Exchange 2007 SP2 installed on SBS 2008, below are the steps to complete from Microsoft Support

http://support.microsoft.com/default.aspx?scid=kb;EN-US;973862