Hello Njan, and welcome to Seven Forums.
If you are referring to doing a repair install
on your Windows 7, then it must be done from within Windows 7 while you have it fully started. You will not be able to do a repair install at boot.
You may be able to use the tutorial below to help extract and replace any corrupted system files from your Windows 7 installation disc to hopefully fix them all though. Extract Files from Windows 7 Installation DVD
Hope this helps,