Use an old PCI card to boot your system while your AGP card with the corrupted BIOS is still installed. Your system should initialize the PCI card first. If this does not happen, remove the AGP card and turn the computer on with only the PCI card, go to your BIOS Setup and change the "Init Display First" option to "PCI". Now reinstall the AGP card. Your system should boot normally with the PCI card as primary display adapter. You can now re-flash the corrupted BIOS or use another one. Make sure you are flashing the right card by using the -i (ati) or -a (nvidia) command line option.