Cpu sudden shut down

Hi everyone
I use a 5 year old system(Desktop). Recently the cpu started shutting down automatically .When i try to restart it logs on but again the cpu gets off after a few minutes. I see the led stops glowing and fans stop working when i mean cpu shutting down. But i see the power to monitor and keyboard and mouse is still there.
Every time i open my system, I couldn't use it for more than 20 minutes. I couldn't turn it on immediately again   (It can't stay for more than 5 min). However i tried to run to some temperature tests through speedfan software.

I have already tried removing the ram chip(I have only one gb) and putting it in the other slot.still the problem persists.
Here are the image links. Sorry, I didn't know any other way to share this pics.
https://www.dropbox.com/s/urwfqexqtuvgiu2/1.JPG
https://www.dropbox.com/s/qf29ob7qghbj841/2.JPG
https://www.dropbox.com/s/amtw53pqomdf631/3.JPG
https://www.dropbox.com/s/gg8e59249jtxb7c/4.JPG

Windows xp
I have intel core 2 duo processor.1 gb ram. gigabyte motherboard g31m-es21.I hear no unusual sounds when it shuts down.I have cleaned my heat sink and turned on the system.
Let me know if u need anymore details. Please Help me