When the BIOS does NOT find the keyboard and can NOT find a boot device, it will turn on legacy USB support and find the keyboard.
What? No it doesn't. If it can't find a boot device it simply states so. the keyboard is found <before> it enumerates available boot devices, and a USB keyboard is only included if both the "enable Legacy USB" is selected <AND> that keyboard is plugged into the
motherboard's USB Port.
Despite the OP's attempts to clarify multiple times their situation you insist that they "remove everything and test the board" despite the fact that
the computer has been doing what I need it to ever since
EVERYTHING so far points to a shot on-board USB host controller. either that or one of your errant suggestions somehow had them disable the on-board USB.
The computer is powering on, but the BIOS is not recognizing any keyboard, therefore their attempts to smash F1 or del or whatever to enter the BIOS are in vain, because the BIOS's "enable Legacy USB support" never applies to PCI USB cards and what is plugged into them.
Eventually windows boots and successfully finds the keyboard and they can use windows just fine.
Suggestions: you've tried a PS/2 adapter- (this is a for a USB keyboard so it plugs into PS/2 right?) try an <actual> PS/2 keyboard. If that doesn't work, then it sounds like your on-board USB host controller as well as the PS/2 controller are both shot.
Only other thing that could cause this is if somehow both of these have been disabled via options in the BIOS, in which case you should reset the CMOS memory by moving the jumper as appropriate- it's usually labelled "CMOS CLR" or something similar, sometimes even "PWD RESET".