Overcoming Error Code 0x800F081F During Office 2021 Installation

Troubleshoot the common error code 0x800F081F during the installation of Office 2021, primarily related to .NET Framework 3.5, by following these structured steps:

1. Manually Install .NET Framework 3.5:
   • Press the Windows key + R to launch the Run dialog box.
   • Type 'appwiz.cpl' into the dialog box and press Enter to open the Programs and Features window.
   • Click on the 'Turn Windows features on or off' link on the left-hand side of the window.
   • Locate and check the box next to '.NET Framework 3.5 (includes .NET 2.0 and 3.0)' and then click OK.
   • Complete the installation by following the on-screen instructions provided by the installer.
2. Use the DISM Tool:
   • Open Command Prompt with administrative privileges.
   • Type the following command: DISM /Online /Enable-Feature /FeatureName:NetFx3 /All /LimitAccess /Source:X:\sources\sxs (replace 'X' with the letter of your installation media's drive) and press Enter.
   • Wait for the command to execute completely and then restart your computer to apply the changes.
3. Run Windows Update Troubleshooter:
   • Access the Settings app via the Start menu or by pressing Windows key + I.
   • Navigate to Update & Security > Troubleshoot.
   • Select the Windows Update troubleshooter and click on the 'Run the troubleshooter' button.
   • Follow the prompts to allow the troubleshooter to find and fix issues related to Windows Update.
4. Temporarily Disable Antivirus Software:
   • Access your antivirus application and navigate to the settings to find the option to disable it temporarily.
   • Attempt to install Office 2021 once the antivirus is disabled.
   • After the installation, remember to re-enable your antivirus software to keep your system protected.
5. Perform a Clean Boot:
   • Press the Windows key + R to open the Run dialog box.
   • Type 'msconfig' and press Enter to open the System Configuration window.
   • Go to the Services tab, check the 'Hide all Microsoft services' box, and click on 'Disable all'.
   • Switch to the Startup tab and click on 'Open Task Manager'.
   • In the Task Manager, disable all startup items, then close Task Manager and click OK in the System Configuration window.
   • Restart your computer to apply these changes and attempt the Office 2021 installation.
   • After the installation, return to System Configuration to re-enable the services and startup items, and restart your computer again to return to normal mode.

Implementing these steps in order should help you successfully install Office 2021 while resolving the .NET Framework 3.5 installation error.