The client firewall needs to be confiured. Please refer to either of the articles below for more info:
"Known post installation event errors in SBS 2008 (and how to resolve them)"
http://blogs.technet.com/sbs/archive/2008/08/26/known-post-installation-event-errors-in-sbs-2008-and-how-to-resolve-them.aspx"Windows SBS 2008 Known Post Installation Event Errors"
http://support.microsoft.com/kb/957713Hope this helps!