This is the bios update tool from the Asus site:
There should be a manual you can read about updating the bios.
I suspect if you put that file after reading about it on a floppy and going to bios at bootup (usually del), there should be a simple update option there
Hope this helps
EDIT: Oh damn, you can't... that is what you should be able to do though, but thats a bit weird if it won't even let you flash because of the CPU in the first place... oh dear...
As long as you have this bios already installed on the motherboard:
A8V Deluxe - ver 1011 (then that CPU should work on that board though this is the latest bios version)