the bios is the chip that controle your computer in many ways.
it is the one that tells witch bootable devise the computer should boot from first.
and it is controling the cpu speed. there are many things that it controles.
if your bios chip brakes, the computer will not work at all.
when you start up the computer, before it starts windows, then press F1. or Delete.
it depends on the motherboard witch key to press. it might also be F2.
here is a video totorial.