Start the computer in SAFE mode. Keep hitting the F8 key. Choose to only boot with the least amount of drivers and hardware.
If you have MSI Afterburner, uninstall it in safe mode. If you do not have MSI Afterburner, remove the video driver and all related software.
Then boot the computer as normal. Download and install a free program called Display Driver Uninstaller.exe - that program will completely clean up driver downloads from AMD and Nvidia. Get it here:
http://www.majorgeeks.com/files/details/display_driver_uninstaller.html
It sounds like your power supply is too weak to drive the video card, especially when overclocked.
EDIT: Remember to download and install new video drivers after uninstalling old drivers.