You need to have a flash drive or CD-ROM disc that you can copy the BIOS to. If you use flash drive, make sure that it is formatted in FAT32.
Go here: http://www.asus.com/Motherboards/AMD_AM2Plus/M3N78_PRO/#download
Select BIOS then use the latest BIOS version and click the Global link (one with green disk). Extract the BIOS file to USB flash drive or burn to cd. Boot the PC (with USB/CD in) back into the BIOS and go to Tools then EZ Flash 2. Use TAB key to change to the proper drive (if not default) and the BIOS file should appear. Select it and follow the on-screen instructions. This is most like what is causing the problem.