HI I have AMD sempron 2800 processor and 512 mb ram , PC works fine on onboard graphics but when I install 128mb nvidia geforce graphic card , PC reboots automatically after every 10 minutes or gives a blue screen error , so is there a problem with the graphic card I am using...

Remove the new graphics card and driver and see if computer operation returns to normal. If it does, the graphics card was incorrectly installed, had the wrong driver installed, or was faulty. If the computer does not return to normal, something was damaged during graphic card installation or the behavior is another issue unrelated to the graphic card change.