Original post : F8 Key - Guide for Windows XP, Vista, 7, 8.
You need to boot into Windows 8. If you can't boot, you need the original Windows 8/8.1 installation DVD/USB - read F8 on Windows 8.
msconfig
If you're looking to use the F8 key to boot into the Advanced Boot Options menu, go to Advanced Boot Options below.
System Configuration You may need to restart your computer to apply these changes. Before restarting, save any open files and close all programs. [ ] Don't show this message again. Restart | Exit without restart
Windows Boot Manager Choose an operating system to start, or press TAB to select a tool: (Use the arrow keys to highlight your choice, then press ENTER.) Windows 7 Windows 7 (Safe Mode) To specify an advanced option for this choice, press F8. Tools: Windows Memory Diagnostic ENTER=Choose TAB=Menu ESC=Cancel
These solutions apply for Windows Vista and Windows 7. For Windows 8 systems, go to F8 on Windows 8.
bootrec /FixMbr bootrec /FixBoot bootrec /ScanOs bootrec /RebuildBcd