You have 1011 so 1015 looks like the final upgrade. It is a Beta BIOS.
You will also need the Flash Utility.
Read all the instructions (Read Me) files and know what you are doing prior to doing it.
BIOS flashes are easy as long as you know what you are doing.
They are potentially destructive if you do not.
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The other possibility is to upgrade the VIA 4 in 1 chipset drivers but ASUS does not list any for XP. You could try these:
http://www.viaarena.com/default.aspx...D=1&CatID=1070
Choose the second option:
Retro chipset VIA 4in1 drivers
Probably easiest to try this first.