I once met the same problem because the SP1 I installed is beta version. Could you remove the sp1 and install the official sp1 and then reboot your machine, then try to install sql again.
Also, I suggest you read this blog .
SQL Server 2008, Visual Studio 2008 SP1, and .Net Framework 3.5 SP1 explained....
http://blogs.msdn.com/psssql/archive/2008/08/11/sql-server-2008-visual-studio-2008-sp1-and-net-framework-3-5-sp1-explained.aspx
Hope this article by Microsoft PSS SQL Support team is helpful for you on SQL 2008.
Dannol Liu