First I would see if this problem is not because the PC is overheated, or the PSU is bad. Use your PC with the housing open, so you can see if the CPU, GPU (graphic card) and PSU fans are all running. If it still shuts down, look quickly all the fans to see if they were spinning. If everything looks OK, the next suspect is the PSU, a quality PSU is a must these days.
BTW most modern boards will allow you to flash the BIOS within Windows with the correct program, or saving the new BIOS in a floppy and flashing with the BIOS (Alt+F2 during POST or something like this).
And welcome to PCTT