Asus P4B266-C Boot Problem - Won't Scan Boot Devices

Hey all! This is kind of urgent as it's for a client of mine so I
hope someone knows the answer. He has an Asus P4B266-C with a P4
1.8Ghz and 512MB DDR266. The system has been working fine for quite
some time, but recently developed a problem. The system goes through
the POST procedure fine and gets to dispalying PCI devices. However,
the system halts there. It doesn't start seeking boot devices though
the floppy light remains on from the seek the BIOS does. This just
starting for my client suddenly two weeks ago. Yesterday, I resolved
the problem by clearing the BIOS and redoing the settings. I also
flashed the BIOS from 1002 to 1006. It worked for a day and now it
has started doing it again. Has anyone seen this before and does
anyone know what causes it or how to fix it? When his system boots,
it runs fine with no instability whatsoever. However, now it isn't
booting at all. Can anyone help me out?! Thanks all!