You can still update in DOS by using a Windows 98 Bootdisk and putting
the flash utility and BIOS a floppy too. I would recommend trying the
latest version of the software for Windows XP as it works flawlessly on
my ASUS P4P800 and XP with SP2. Here is a link:
http://www.asus.com/pub/ASUS/mb/flash/Asusupdt52801.zip
Also, for your ASUS P4P800 Deluxe, you are going to want to update as
there was a recent BIOS release that fixes the issue with a USB device
that is connected to the system is put as the first in the boot order.
I am still waiting until they release this for the P4P800 (non Deluxe).
A little secret if you like overclocking. You can flash the BIOS with
the BIOS from the P4P8X (flashing the ENTIRE BIOS) which will limit the
processor to a FSB of 533. Now flash back to the P4P800 Deluxe BIOS
(but do not use the command to overwrite the ENTIRE BIOS, just to flash
the normal parts). This leaves the bootstrap of 533 and allows you to
enable PAT at FSB higher than 200 MHz
Running with Full PAT at 250 MHz on the FSB right now and it is great.