Why would you try to repair Windows 7 by installing Win8 thereby wiping it out? Do you have Win8 upgrade license you'd prefer using? If so you should burn the ISO using an image burner, Clean Install with Windows 8 Upgrade
If not is there a possibility Windows 7 is still on the HD, or did Win8 install pass the phase where files were expanded?
If in doubt then use the Disk provided in Step 5 of Troubleshooting Windows 7 Failure to Start
to boot into Windows 7 Startup Repair and see if it finds an installation to repair. If so try the repair steps in tutorial to start Windows 7.
Otherwise use Step 9 rescue method to browse in further to see if you can rescue any files that might be moved to C:/windows.old in stranded Win8, or elsewhere.
Then skip to Clean Reinstall - Factory OEM Windows 7.