You can use the Asus AI Suite II that comes on the driver disk or you can download it from the motherboard driver download page from the Asus website and use that to update the BIOS from within Windows.
Just download the BIOS file extract it to the desktop, open the AI Suite and choose update, there is then an option to update from file, just choose that and select the BIOS file from the desktop. Or there is an option to update from the internet although for some reason that has never worked for me.