Try the F5 key and see if that does it. F8 doesn't work with an Asus motherboard, at least with the American Megatrends BIOS; they made it F5 instead - for who knows what reason. Also, if the F5 doesn't help on your PC, try waiting a bit longer in the bootup sequence. Like after the bootup screen - checking devices, etc. - appears, then hit F8, several times, if necessary.
I also discovered, when I found the F8 key problem, that once you have a USB keyboard or mouse installed, P/S2 mouse will no longer work. (Not sure about the keyboard, though) Wasted a measly $5 on a Logitech serial mouse.
Worth a try or two.