Issue:-

Error 0x800f0818 while installing Service Pack1 on Windows 2008 R2 x64

Cause:-
Dot net framework patch corruption (KB2518867)

Steps taken to resolve the issue:-

Check the "CheckSur.log" file located at "%windir%\Logs\CBS"

You may see the below error in the files.

Unavailable repair files:

 servicing\packages\Package_for_KB2518867_RTM~31bf3856ad364e35~amd64~~6.1.1.0.mum

 servicing\packages\Package_for_KB2518867_RTM~31bf3856ad364e35~amd64~~6.1.1.0.cat
Once you have the files which are causing the issue then download the patch, in my case, I have downloaded KB2518867 patch.Windows6.1-KB2518867-x64.msu

Launch the command prompt in Admin mode; extract it to “C:\temp” location by running the command "Windows6.1-KB2518867-x64.msu /extract"

Once extracted, now extract the “Windows6.1-KB2518867-x64.cab” file in the “C:\temp\KB2518867-Cab”.
Now we have to take the ownership and change the permission.

Take Ownership

takeown /f C:\Windows\servicing\Packages\Package_for_KB2518867_RTM~31bf3856ad364e35~amd64~~6.1.1.0.mum

takeown /f C:\Windows\servicing\Packages\Package_for_KB2518867_RTM~31bf3856ad364e35~amd64~~6.1.1.0.cat

Change Permission

icacls C:\Windows\servicing\Packages\Package_for_KB2518867_RTM~31bf3856ad364e35~amd64~~6.1.1.0.mum /GRANT Administrator:F

icacls C:\Windows\servicing\Packages\Package_for_KB2518867_RTM~31bf3856ad364e35~amd64~~6.1.1.0.cat/GRANT Administrator:F

Now replace the below two files from “C:\temp\KB2518867-Cab” and paste it to “C:\Windows\servicing\Packages\” 

Package_for_KB2518867_RTM~31bf3856ad364e35~amd64~~6.1.1.0.mum

Package_for_KB2518867_RTM~31bf3856ad364e35~amd64~~6.1.1.0.cat

Restart the Windows Update service
Issue Resolved!!! :-)