Sounds like a driver problem. Go to safe mode and delete and install new driver. Your new driver must be 100% compatible with your operating system! For example if you have Windows 7 64-bit version you must have the corresponding graphic drivers for that Windows! Go to nvidia website and select your graphic card model, your operating system version and e.c.t. then download and install your new driver. Restart your computer. That will (probably) fix your problem.
And to be absolutely sure that it's not a power supply issue, give me your psu (full) brand/model name, and i will check it for compatibility.