I don't know, if you can see the boot-time scan screen (see my previous image) ?
Then can you schedule it again and when it starts, click the Esc (escape) key, if that aborts the scan then your keyboard is being recognised and it could to be something else, what I'm unsure.
It won't hurt to try to interrupt the boot to try and enter the BIOS settings, but if your USB keyboard isn't set-up you may not even be able to do that. It is a chicken and egg situation, if your keyboard isn't recognised prior to windows boot then you can't get in to adjust the settings.
How old is your system and did the USB keyboard come with it ?
As has been said if you can borrow a ps2 keyboard that will certainly work if the same problem occurs then it isn't keyboard or BIOS related.
You say you were going to try a reinstall but didn't report the result, I assume because you are still chasing it didn't work ?
Download the avast! Uninstall Utility,
find it here and save it to your HDD so you can find it later.
Now uninstall, reboot, run the uninstall utility, reboot, install, reboot.