Checksum error in bios

Question

asus m3n78em motherboardhthe computer has been working for a while with bios 0501 for a while but the other day it would not boot. i clreared comos and retried but it came up with bios checksum error and started searching for the bios to reload via floppy or cd or usb.i had the bios on usb and plugged that in and retried but it came up with the same bios checksum error. i then tried with the original cd withe same result. any ideas please