hey i have faced a problem like this many times!
i am not sure but its u r graphics card problem!
Disable overclocking!
first of all uninstall any grapic drivers u have in safe mode and then reboot.
get the latest drivers from here
http://www.nvidia.com/content/drivers/drivers.asp
get in safe mode and install the drivers !
THERE MUST NOT BE ANY KIND OF ANTI-X TOOL RUNNING!
After installation let the PC reboot!
Then boot in safe mode!
And in case u are having overheating problems get another fan!
or contact u r technician!
Or use tools like fanspeed or Motherboard monitor!
Hope this solves u r problem!