Display problem

Recently i am having a strange problem. computer starts but there is no display in the monitor. after few hours later when i power on the pc it starts with display. but when i restart there is no display again. first i have thought the problem might be in the monitor. then i have boutht a lcd monitor(benq). but few days later the problem reappears & still continuing. while starting the computer i do not hear any kind of beep code. i have also plugged vga cable to my built in graphics card, but the problem remains. my pc configuration is------

processor: intel pentium D 3.2GHz

hard disk: 320GB

RAM: 2 GB

Graphics card:nvidia pci express 256 MB