There are any number of things that could cause this. A graphics driver problem, or conflict might cause this, but if the machine is almost new, the newest drivers for your card are probably focused on supporting Vista/7 not XP. If a new driver does not help trying a older driver might even do the trick.
Beyond that the next suspect would be something faulty with the GPU. Do you have integrated graphics or a separate card? A problem with the GPU might explain why you have to hard reboot to get everything back to normal.
Over heating could cause such a problem. A faulty or overtasked power supply. Maybe even bad ram.
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