It's strange when you say F5 fault code. If I am right, this F5 fault isn't an error but an instructions/directions for you to proceed forward with something. Changing your cmos battery will not fix this. Actually there must a message like "Press F5 to..." if you can let me know that, it would be much helpful for me to assist you. Well, first follow the instructions below; Disconnect the power cable, display/vga/dvi cable, all the peripherals like printer, usb, speaker, etc.. after power off the computer. Now, disassemble the pc by removing those screws on the cabinet, motherboard, etc ... etc.. Remove the Rams, Cpu, disconnect the inter cables like front panel, usb, speaker, power, ide/sata, etc.. and if you get time, clear everything on the motherboard with a soft brush. Again assemble the pc and reconnect everything and install all the hardwares like rams, cpu, etc.. and reboot your system, and let me know. Good luck

Hi! The board may have blown capacitors or leakage, the reason why it fails to boot. Look for blown capacitors and replace it. After the procedure you'll be good. Hope this helps and good luck!