The installation of .NET 3.5 can fail for several reasons. Sometimes the .NET 3.5 install fails and sometime SQL Server Setup fails to detect .NET is already on the machine.
If you view the %temp%\sqlsetup.log you can determine whether the .NET 3.5 installation failed or SQL Server Setup incorrectly detected .NET was already installed.
Here is an example were .NET 3.5 SP1 failed to install.
05/21/2008 07:32:55.750 ======================================================================
05/21/2008 07:32:55.765 Setup launched
05/21/2008 07:32:55.765 Attempting to determine media source
05/21/2008 07:32:55.765 Media source value not specified on command line argument.
05/21/2008 07:32:55.765 Setup is launched from media directly so default the value to the current folder.
05/21/2008 07:32:55.765 Media source: \\sqlbuilds\Improvements\clusterRefresh\10.00.1441.12\retail\SQLFULL_ENU\
05/21/2008 07:32:55.765 Attempt to determine media layout based on file '\\sqlbuilds\Improvements\clusterRefresh\10.00.1441.12\retail\SQLFULL_ENU\mediainfo.xml'.
05/21/2008 07:32:55.796 Media layout is detected as: Full
05/21/2008 07:32:55.796 Local setup.exe not found, so continuing to run setup.exe from media.
05/21/2008 07:32:55.796 /? or /HELP or /ACTION=HELP specified: false
05/21/2008 07:32:55.796 Help display: false
05/21/2008 07:32:55.796 Checking to see if we need to install .Net version 3.5
05/21/2008 07:32:55.796 .Net version 3.5 needs to be installed from media: \\sqlbuilds\Improvements\clusterRefresh\10.00.1441.12\retail\SQLFULL_ENU\
05/21/2008 07:32:55.796 Attempting to find media for .Net version 3.5
05/21/2008 07:32:55.796 Attempting to install .Net version 3.5
05/21/2008 07:35:36.725 Received exit code -1073741818 from installing .Net version 3.5
05/21/2008 07:35:36.725 .Net version 3.5 installation failed, so setup will close. Error code: -1073741818
05/21/2008 07:35:36.725 Setup closed with exit code: 0x84C40013
05/21/2008 07:35:36.725 ======================================================================
When .NET failed to install, your next step is to look at the .NET log files. The logs produced by the .NET Framework 3.5 setup wrapper:
You should review these log to determine the cause for failure. If you still need help look at these resources:
- Post the error to http://social.msdn.microsoft.com/Forums/en-US/netfxsetup/threads
- How to: View SQL Server Setup Log Files: http://msdn.microsoft.com/en-us/library/ms143702.aspx
- Troubleshooting an Installation of SQL Server: http://msdn.microsoft.com/en-us/library/ms143698.aspx