About 1 in 5 times I see this, this is fixed by disabling the page file, rebooting and running SFC, and enabling the page file again. If you look through the CBS log, it is helpful in finding what its complaining about. If you use an automated cleanup program, it can remove redundant system files which do nothing but take up space (like language packs) but produce SFC failures. Also on domain machines with a WSUS server, it will fail just about every time because you have to set in local group policy to check Microsoft update servers for optional components and repair.