Although you already have VS2008 SP1 installed.

Fix:

start -> run -> "cmd"

Setup /ACTION=install /SkipRules=VSShellInstalledRule

you will still get an error but it installs fine.

The SQL Server 2008 installer is checking the HKLM\SOFTWARE\Microsoft\DevDiv\XXX\Servicing\9.0\SP reg keys to tell if SP1 is installed. If you inspect those keys and find any SP keys with the value “0,” that is your road block.