New motherboard:Can’t detect KB or mouse

I’m rebuilding a computer for my daughter. I installed a new power supply, new floppy drive, new 60 gig HD, and lastly; a new motherboard with AMD Athlon 2400 processor. Now, when I try to boot up to install the OS, the computer doesn’t detect my keyboard or mouse, so I am unable to proceed at all and it goes to a blank screen..
Any ideas?????