On most of my older XP machines hitting F8 got me straight to the safe mode screen but not this one, it goes to a drive selection first and you have to hit F8 again to get to the screen. Then clicking on that would launch memtest or what ever I was trying to run from the flash drive. So I would hit F8 continuously and the selection screen would show and bingo on the selection screen was listed my usb flash boot drive. What I didn't realize at the time was that's where I wanted to be to boot from my usb flash drive.
Typically when I go into safe mode for a system restore or something I would then select my main drive and then click on the F8 key again very quickly and that would get me to the safe mode selection screen. When I do that I first get a screen that asks which drive I want to use. I don't know if its the motherboard, bios or the OS but what I have to do to boot from usb now on this machine is to start like I'm going into safe mode (F8 in my case). Even when I set my bios to boot from other or removable drives I do not get the press any button to boot from usb anymore on one of my machines that had Vista and then Win7.