Make sure you back up everything. You can never be too careful.
If its a zip file, open it and save the contents to a floppy drive.
You will now need a windows boot disk. Preferably windows 95 or 98, just to make sure that nothing else is running in the background that can ruin the Bios update. I've run a windows millenium boot disk for this purpose and it worked fine.
You load the windows boot disk when u start up.
After it boots it, put in the bios disk.
Unless Asus says to do something else, I believe all you have to do is type in "flash" and it should work automatically.
Before you actually try this. Check Asus's site to make sure what the correct steps are to do a bios change. That would probably be the best thing to do. Its probably in their FAQ site.