To figure out the cause we'd need your VS 2008 setup install logs. You can gather these using the Collect tool: http://blogs.msdn.com/astebner/archive/2007/08/01/4172276.aspx
In the meantime, you should be able repair your install by running:
"%windir%\Microsoft.NET\Framework\v3.0\Windows Communication Foundation\ServiceModelReg.exe" /r /x
%windir%\Microsoft.NET\Framework\v3.5\WFServicesReg.exe /c
Hope that helps,
Dan Glick
Software Development Engineer, WCF