Safe mode with command prompt does not work

First test the hard drive with SeaTools for DOS: http://www.seagate.com/support/downloads/seatools/

You can make a bootable CD from the ISO file with IMGBurn on a working computer: http://www.filehippo.com/download_imgburn/

Boot your faulty PC from that CD and the testing software will open. Test the drive with the long/extended test.
If the drive fails the test, you will have to replace it.

If it passes the test, re-install Windows on it or perform a factory recovery, whichever is possible.