Not sure whether this is any help, but I had a similiar problem with my computer crashing to desktop, once right in the middle of a clan match
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I have since defragged my hardrive, cleared out all the old temp files etc, just general maintenace, and so far it has been fine.
With regards to the white artifacts your describing I had a similiar thing with my old GEforce 3 card when the fan started to fail and it was overheating, might be worth looking into.