I agree, you don't need to run both power supplies. Even if you were running SLI as I am, you still would only need less than a 1000w. My system uses just over 800w at full load.
As for your problem, I also think it's your driver. The newest driver from Nvidia, 196.21 as well as all the other 190 series of drivers are crap. What's worse is after un-installing the 196.21 driver and running Driver Sweeper you'll still have leftover files. Look for a Nvidia folder in your Program Files and then look in Windows\System32\ and delete nvmctray.dll-nv13568 and nvapi64.dll-nv13568 (this file maybe named slightly different with a 32-bit OS). Once that is done install the 186.18 drivers from Nvidia.
Driver Sweeper Guru3D - Driver Sweeper (Setup) download from Guru3D.com
Win XP NVIDIA DRIVERS 186.18 WHQL
Win XP x64 NVIDIA DRIVERS 186.18 WHQL
Vista/Win7 32-bit NVIDIA DRIVERS 186.18 WHQL
Vista/Win7 64-bit NVIDIA DRIVERS 186.18 WHQL
As for your problem, I also think it's your driver. The newest driver from Nvidia, 196.21 as well as all the other 190 series of drivers are crap. What's worse is after un-installing the 196.21 driver and running Driver Sweeper you'll still have leftover files. Look for a Nvidia folder in your Program Files and then look in Windows\System32\ and delete nvmctray.dll-nv13568 and nvapi64.dll-nv13568 (this file maybe named slightly different with a 32-bit OS). Once that is done install the 186.18 drivers from Nvidia.
Driver Sweeper Guru3D - Driver Sweeper (Setup) download from Guru3D.com
Win XP NVIDIA DRIVERS 186.18 WHQL
Win XP x64 NVIDIA DRIVERS 186.18 WHQL
Vista/Win7 32-bit NVIDIA DRIVERS 186.18 WHQL
Vista/Win7 64-bit NVIDIA DRIVERS 186.18 WHQL