I think either this is a bad sector, or something which had happened to me before -
I had got some files missing. So i ran the repair console and fixed it. Then after 1 - 2 restarts it happened again.
Eventually i found out that sometimes its a virus which keeps on deleting your important system files. If you copy these files once, the pc will run until the virus is active again and affects them again. Best way to do is, startup your pc once by copying the new file, then use any antivirus and clean up your pc. If it still doesnt clean the virus then you've got to use the formatting option, or seek professional help