I thought the atitool was for ATI cards. Your post has the gtx 285.
You didn't reveal the connection to the HDD, if IDE you know the XP DMA issue and cure.
i have c2d e6600 cpu, gtx 285 gpu, 4gb ram, and everything worked fine till recently... on every 3d application cpu usage is 30-60%... i discovered this only few days ago... before that i had some fps issues, tried to update gpu driver and got this error in event viewer "The file system structure on the disk is corrupt and unusable. Please run the chkdsk utility on the volume C:." event id is 55... after that, chck disk did it's job and after reboot i installed gpu drivers(booted 640x480res)...
i checked it for viruses with several tools. it's clean...
tried prime95 to torture cpu and ram. after 12 hours no warnings or errors.
also tried manufacturer diagnostic tool for hdd. it's healthy.
tryed atitool for gpu test... no artefacts...
any idea is more than welcome