Try booting from the cd again. When it asks to install Windows select yes. It will then look for existing operating systems and give you the choice to repair existing or install new. If you do not get this option then the cd is a different version , home or pro, to the installed version. Borrow a cd of the correct version and you will get the option to repair BUT you will need the authentication code. You may get past the first request for it but after a successful repair you may get stuck at a login screen saying this copy must be activated before logging in. If all the drivers are present and the pc will connect to the internet it may activate automatically.
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