Can`t answer that, my initial thought is to check that the bios is open to legacy usb devices - if it can do that.
My other thought is that prehaps u`r trying to instigate the bios on boot up and are mashing the del button to get in - usb devices will submit bad info to (particularly older) motherboards and even if u manage to get into the bios, u lose control of the keyboard when u`r in there - reset being the only option. In these cases u press del once or 2ce when it says on the screen and sit back until it`s invoked the bios menu - then u can use the keyboard.
If u`r mashing - it`s hit/miss if it`ll give u control, otherwize I have no idea >.< other than to ps2 it ?!?!