When I launch the SP1 update, installation fails with error 0x800f0818.
Here are the step used to solve this issue:
1. I have downloaded following Microsoft Tool:
http://support.microsoft.com/kb/947821
The tool runs and checks that everything is clean for an upgrade to SP1.
2. If it doesn't fix anything, it just checks and leaves logs here:
%SYSTEMROOT%\Logs\CBS\CheckSUR.log
3. If you see a number of fixed files at the top of the log: try to install Service Pack 1 again.
4. If you see anything under "Unavailable repair files": keep reading.
In second case if you get warning about corrupted file:
Package_for_KB2506014_RTM~31bf3856ad364e35~amd64~~6.1.1.0.mum and Package_for_KB2506014_RTM~31bf3856ad364e35~amd64~~6.1.1.0.cat.
5. From Microsoft download patch. Open a browser window and navigate to
http://support.microsoft.com/kb/(number here)
and 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.
6. Copy the .mum and .cat files to "%windir%\Temp\CheckSUR\servicing\Packages" and run tool again