MigAce32,
Although flashing under windows may work most of the time, there are a lot more things that can go wrong.  My advice is to only flash using a boot disk with no drivers loaded.
Having said that, you will need to go to ASUS' website and download their AFLASH.EXE ultility along with the most recent BIOS release for your board.  Copy them both to your floppy disk and then reboot your computer using the boot disk.
It is ALWAYS a good idea to save your current BIOS to the floppy disk before writing the new BIOS.  Using AFLASH is pretty straight forward.
Couple of cautions:
1) No matter what people say, there is always a chance that you can ruin your motherboard when flashing it.  Although there are services that may be able to help (
http://www.badflash.com), they are no guarantee.  You need to be aware of this before starting.
2) Once you have started the BIOS erase phase, your MB is dead unless you can program it successfully.  This means that if you lose power, turn the machine off or something bad happens between when the erase starts and the program finishes then you will have a problem.  As a result, if something goes wrong, you will need to do whatever possible not to turn the machine off.  This is why it is so important to save the exisiting BIOS to the floppy before starting.
I have flashed many boards and only had one failure.  The machine hung completely after finishing the first part of the programming phase.  I was lucky:  BadFlash was able to write me a new BIOS chip that worked when I installed it.
Good luck.