It is very likley you have you a virus. One that is corrupting your files. If you have and antivirus I suggest doing a full or deep scan as soon as possible becasue it is likely for that virus to spread to your OS and corrupt start up files and eliminate your whole booting system. This will mess up your system to the point of needing to reformat the whole system. After your virus scan. And after you elimate the viruses, pop in your operating system disk and just repair the missing files.