Don't know if this will help for Win 8.1 but it's what I had to do for repairing a Win-7 partition once.
Go into the BIOS to change the boot device to USB and insert USB drive, then restart system.
In 'Windows Setup' dialogue set the Time and currency format, Keyboard or input method and the language to install and then click 'Next'.
Click on 'Repair your computer' from the lower left corner.
From the 'Choose an option screen', click on 'Troubleshoot'.
Click 'Advanced Options' in the 'Troubleshoot' screen.
In the 'Advanced Options' screen, click 'Automatic Repair'.
Follow the on-screen instructions to complete the Automatic Repair.
Good luck!