why does the system often get restarted automaticlly?

[yz wang]

Hardware of my computer: Pentium 4 cpu 3.20GHz, RAM 2.00GB, Monitor NVIDIA
GeForce FX 5600, two 80GB harddisks, one with windows and one  with fc6.
3D desktop is off.
It often crashes or restarts. The latest restarting occurs while only 3 web
browsers are open. How can I solve this problem? Thanks!
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