If you encounter this error:
Checks for previous releases of Microsoft Visual Studio 2008. A previous release of Microsoft Visual Studio 2008 is installed on this computer. Upgrade Microsoft Visual Studio 2008 to the SP1 before installing SQL Server 2008.
while installing SQL 2008 even thou you have the latest Visual Studio 2008 SP 1. You can bypass this check by modifying this registry key (simply rename to anything you want):
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\DevDiv\VS\Servicing\9.0
Hope this helps you save some time troubleshooting.