Well the F8 Windows boot options would be seeing the Safe Mode with Command prompt option which is what that sounds like to me! You still got into the Windows boot options not the bios boot device menu one of the F keys is supposed to bring up before any part of an existing Windows installation starts loading.
Upon mentioning you are trying to see a full clean install of 7 go on you have two choices you can make to insure seeing a boot from the usb flash drive being changing the default boot device in the bios setup or bringing the one time only boot device menu where after a restart you wiil revert back to the hard drive or device already set as the default first in the boot order. Going into the bios to change that will insure you see the boot off of the usb key but also mean once the Windows setup files have completed being copied over you have to go right back in there to reset the hard drive as the first like it had been.
One problem you could be running into if you are not seeing the F key you would use like F12 to bring up the bios's own boot option menu where you choose the drive or device like flash drive or external hard drive, memory card to boot from would be the UEFI issue mentioned in the clean install keys here at SF, over at the Eight forums where this came up much more so, and the Ten Forums of course for the newest version out.
The UEFI is the Unified Extensible Firmware Interface offering the Secure boot option in the bios setup that can prevent booting from a flash drive. The guide you would want to go over on that is seen at
http://www.sevenforums.com/tutorial...e-firmware-interface-install-windows-7-a.html
Now another question I have for you of course would be the make and model of the mother board you have there so I can look up the user manual as well as specs to see how the bios goes as well as the boot options that are available with that model MB you have there. I suspect with your mention of a clean install you are planning to wipe the other OS off of the drive as well?