I just build a computer with:
ASUS A7N8X deluxe , Athlon 2500
radeon 9100
2-256 Kingston sticks
XP Pro
The computer works for 2-3 weeks, then last night I do some windows update now it crashed saying when I boot up the computer : system memory test fail! I cannot get back to the Bios . Is the motherboard is dead ? Please help me and what I should do. Thanks ina advance.

Is this a you-built-it (ordered individual parts) or a store-bought machine (a bare-bones setup and added additional parts)?

If it is the second you may have some additional warranty options, if there really is a major problem.

Now, on to the problem......

The first thing to try is to reseat the memory.

If there is no change and the RAM doesn't need to be instlled in pairs, then try a single stick of RAM. Also try this RAM in another compatible machine and other REAM in this machine.