The safest and most reliable way to do this is with a 3.5" floppy disk. When you build a new PC its worth the 15 bucks for the drive and cable to install just in case this type of situation arises. Even the motherboards that are supposed to give you other options don't always work or you end up hosing the bios. IF you have the 3.5" drive then its easy to find a boot dik image on the web from a DOS 6.x or Win95/98 to get the machine booted to run the flash utility and update the BIOS.

If you don't have the 3.5" floppy drive then you are fucked.

This might help though:

BIOS Flashing with a bootable CD-Rom