The opportunity to enter the BIOS should come up before Windows attempts to start.
The default is to press the Del key as the computer first boots up. On some models it may be different -- the answer then is to press load of keys and the BIOS will kick up a message about keyboard error and offer Enter BIOS as an option.
If you are getting nothing at all on the screen you need to think about other possibilities -- are you hearing beeps, are the keyboard LEDs blinking, is the monitor attached and working etc.