|When in a pre-Windows mode your mouse usually does not work. There are a number of keys that can be used to maneuver in that environment.|
Pg up/dn, arrow keys, enter, tab, pause, scroll Lock, backspace, F keys.
To select start options the Pg up/dn or arrow keys should work, followed by the enter key.
If nothing works at that time you may be using a USB or wireless USB keyboard. It is possible to make a USB keyboard function in that environment but requires a wired PS/2 keyboard to enter the BIOS and select to Enable legacy USB devices.
If your computer did not come with a PS/2 port then you need to use the original USB keyboard that was shipped with the computer.