أعتذر للكتابة بالإنجليزي لكن هذه الخطوات عادة ما تكون بالإنجليزية
Restart your PC and make sure that operating system CD inside
Boot From CD
After installing installation files select installing operatin system
Then the setup will list the operating system you are using
Select R for repair
Then let system continues repairing and do not do anything unless it prompts you to do so as for regional setting or operating system key
The above is just to repair your system without touching your files.
do not listen to those who will advise you to format your PC because it is the easiest thing to be done