Dell optiplex 790 loop start

I have my Dell Optiplex 790 machine it was working accurately with 250 hard, 4 GB Ram, Core i5 3.1Ghz processor 5days before processor's fan ran with abnormal sound, after 3 days when i powered on my computer it took restart after 40-45 minutes of starting up and then it starts to restart with single minute intervals (loop startups) and now today it is 5th day of this prob and it is not giving a display for a single second (takes a startup with interval of 2-3 seconds). Same problem was happened with my friends computer he had also same device with same configurations, he tried to recover with different tries but failed and at last he changed the machine. If any one can help me to solve this prob ?