You need the "Popup boot menu" key.
There are two ways to select a drive.
1) Enter the BIOS and make a permanent change
to the boot order. This is... inconvenient.
2) On my Asus, the BIOS entry key is <Del> and the
popup boot is F8. On my laptop, popup boot is F11.
The popup boot presents a menu of storage devices.
You select the device desired from the menu, and off
it goes. You have to hammer the F8 key at the appropriate
time (so timing is important). If you're too late,
F8 can get an old OS into Safe Mode :-( Depending on
what key they decided to use, and the timing window
involved, this can be super-easy, or moderately
hard to get right. I can never get my laptop to do
the right thing on the first try, and the key entry
window is 1 second wide on it. Your USB key will be
in the menu.
If you disable the "splash screen" in the BIOS, such that
the machine boots "with text showing", there is usually
a single line of info there documenting the BIOS entry
key and the popup boot menu key. There are some computer
manuals which neglect all the details, and then you have
to fend for yourself.
If you mention the model number of your Abit, I can check
my small collection of manuals to see if it is there. I
don't know if there is still an Abit manual archive around
or not.
*******
I can never find an actual good looking example of
a popup boot, and the text in these examples is usually
lame and not like your PC at all. In any case, the
*decorations* around the outside of this BIOS level
window, are always the same. Almost as if it's a
standard of some sort.
"Popup boot"
http://docs.oracle.com/cd/E19127-01/ultra27.ws/821-0171/images/7-2-Boot-Device-Menu.gif
Note that, on UEFI machines (your machine is a bit old
for UEFI), the storage devices can appear twice in the
list. Once with a "let's try UEFI" option for the
storage device, the other with a "let's try legacy BIOS"
option for booting. If you know your OS was installed
in legacy (CSM compatibility enabled) mode, then you would
select the non-UEFI entry in the menu. My single example
of a UEFI machine here, shows two entries for each drive.
The machine I'm typing on, is from the same era as
yours, has a popup boot, but is not UEFI ("New BIOS")
equipped.
Popup boot appeared about a year after the first
USB2 programming window appeared in BIOS designs.
Whenever that was :-) Back in those days, you
could do cool things like change the storage emulation
used for USB2 devices, to suit your purpose. Modern
machines don't bother to do that any more. They're
probably always using some sort of hard drive emulation.
If your machine was missing popup boot, then... this
project is not going to be easy at all. I have three
or four machines without popup boot, so USB flash keys
are going to be a problem. I won't be booting a
Kaspersky USB key on my 440BX based Win98 machine.
(It's got USB 1.1 ports I think.)
Paul