Are you using a USB keyboard? If so, that is likely the cause of your problems.
Use a regular PS2 keyboard, and you should be fine.
If using a PS2 keyboard isnt working, then I'm not really sure what else the problem could be, and therefore wont know how to fix it.
The only other thing I can think of is a CMOS reset, which involves jumpers on the mobo. You can also do the same thing by taking the CMOS battery out for about 15 minutes.