From the indefatigable Matt Quick:
Sometimes, trying to install .NET 3.5 either via Add/Remove Programs or via the standalone offline installer produces an error code. This is due to WSUS not having the files for .NET 3.5. Use the following workaround to avoid taking the machine off the domain, installing .NET 3.5, then putting it back on the domain:
This worked for me. Windows has to download the 3.5 installation files, but the server is configured not to use Windows Update (common for managed servers), but WSUS. The above article describes how to fix this. In a nutshell:
- Start the Local Group Policy Editor or Group Policy Management Console.
- Expand Computer Configuration, expand Administrative Templates, and then select System.
- Open the Specify settings for optional component installation and component repairGroup Policy setting, and then select Enabled.
- Select the Contact Windows Update directly to download repair content instead of Windows Server Update Services (WSUS) check box.
Make sure Windows Updates Service is set to Manual or Automatic to apply this fix.