From the sign-in screen
Restart your PC. When you get to the sign-in screen, hold the Shift key down while you select Power Power icon
After your PC restarts to the Choose an option screen, select Troubleshoot > Advanced options > Startup Settings > Restart.
After your PC restarts, you'll see a list of options. Select 4 or F4 to start your PC in Safe Mode. Or if you'll need to use the Internet, select 5 or F5 for Safe Mode with Networking.
To clean install tap f2 or esc usually on when booting to get into bios, set boot order for the windows installation media required ie optical if disk, if fails to lock the boot order, reboot tapping f12 and select the flash / disk to boot from, on the screen where asks wish install, select custom advanced , i advise format the partitions to remove the previous windows contents , your choice, if do not your previous files are under C bottom folder windows old