If on (fresh installed) Windows Server 2008 R2 you are not able to install SP1 with error code 0x800f0818 in System's Event Viewer (Event ID 8), check this log file: "C:\Windows\Logs\CBS\CheckSUR.log" (after installing KB947821).
Then, maybe you'll want try following steps:
1. Download this x64 KB from Microsoft (Security Update for Microsoft .NET Framework 3.5.1 on Windows 7 and Windows Server 2008 R2):
http://www.microsoft.com/download/en/details.aspx?displaylang=en&id=26289
2. Expand the msu package.
Example:
expand C:\sourceDir\Windows6.1-KB2518867-x64.msu /f:* c:\tempDir
3. Expand the cab file.
Example:
expand C:\tempDir\Windows6.1-KB2518867-x64.cab /f:* c:\targetDir
4. Change Ownership of "C:\Windows\servicing\packages" folder to your account and give for your account full control privilege (you will need it for last step).
5. Replace this files***:
C:\Windows\servicing\packages\Package_for_KB2518867_RTM~31bf3856ad364e35~amd64~~6.1.1.0.mum
C:\Windows\servicing\packages\Package_for_KB2518867_RTM~31bf3856ad364e35~amd64~~6.1.1.0.cat
with the same name files from "c:\targetDir"
*** files for step 5 may be different according to errors in CheckSUR.log file.
