See http://www.winguides.com/registry/display.php/790/ and this Google group search result<-- the user tried repair installation.
Friend was given a PC,(Windows XP), which won't boot up properly, possibly due to virus. Message received is "application has failed to start because sfc_os.dll was not found. Suggests re-installing but this doesn't fix the problem. Can get as far as "Start", "Run" "Accessories" but can't get to Command Prompt, as above message shows again. Don't know how to re-enter missing file. Would eventually like to FDisk and start re-installing all software.
Can anyone help??