Fix Error 0x800F0818 when Installing Service Pack 1 (Windows 7)

1.     Find and download the System Update Readiness Tool for the version of Windows you are working on. The tool can be found on the Microsoft website with a quick Google search.

2.     Run the System Update Readiness Tool installer you downloaded. "Installing" the tool will also run it for the first time.

3.     Open Windows Explorer (Start - Computer) and navigate to "%systemroot%\Logs\CBS\" using the address bar.

4.     Open CheckSUR.log using Notepad or a similar text editor.

5.     If you see a number of fixed files at the top of the log: try to install Service Pack 1 again.

6.     If you see anything under "Unavailable repair files": keep reading

7.    
If there are unavailable repair files you will see two or more packages. As an example: Package_for_KB2506014_RTM~31bf3856ad364e35~amd64~~6.1.1.0.mum and Package_for_KB2506014_RTM~31bf3856ad364e35~amd64~~6.1.1.0.cat.

8.     In the file name should be a number proceeded by "KB" (KB2506014 in this example) which is is the update package that is corrupted. Make a note of the number.

9.     Open a browser window and navigate to support.microsoft.com/kb/(number here) for example support.microsoft.com/kb/2506014.

       10.Find and download the package for your version of Windows on the resulting page. Do not run the             installer.

        11. Use an extraction utility like 7-Zip to extract the CAB files from the MSU installer and then again                    to extract the contents of the CAB files.

        12. Alternatively you can use the Expand -f: command line utility.

• 13. In the extracted files locate the .mum and .cat files mentioned in the CheckSUR.log.
• 14. Copy the .mum and .cat files to "%windir%\Temp\CheckSUR\servicing\Packages"
• 15. Run the System Update Readiness Tool installer again and check the CheckSUR.log file from steps 3 and 4 when it is finished. You should see that the files have been repaired.
• 16. Install Service Pack 1 normally.