Fedora 12 automatically reboots

[akash haibatpure]

I install Fedora 12 on my machine ( configuration RAM - 1GB HDD-160 GB AMD processor)
I used Fedora 2 month it works properly but before some days OS hangs and I reboot system
after that when I try to log in into system, system automatically reboots.

After lot of try I install fresh copy Fedora 12 then also same problem happens with me
i.e. when I logged in system automatically reboots.

Can you explain why this happens and solution for this problem.