My pc gets shutdown automatically.And it take’s long time to start.Help Me?

Hello
I am using
Intel core2 duo E7500@2.93Ghz
Windows 7
Nvidia 8400GS 512MB
320GB HDD
Whenever i start my pc it takes a long while to start.
It restarts minimum 2-3 times and then it start’s
And after it start’s the LCD’s display go off and the lights of CPU stay on continously.
Please help me on it!
Thx