My computer turns off randomly

I just got this computer off cragslist and it will randomly turn off with out a warning. i checked to see if the ram was bad and it is fine. I just installed vista so its not a op system problem. there is no virus. What i think is i need a better power supply. it has a quad core 2.5ghz. i went to the bios and turned off all the cores so it is only running on 2.5 and it seems to be working fine. please help guys cuz i want to be able to use all the cores.

if you need any more information about my computer or what ever i would gladly tell you
 
What is the make and purported wattage of your power supply? Give us your hardware specs.
 
um i am not sure how to find out the power. the hard ware is
intell core 2 quad Q300
the mother board is intel corporatiion dg41tx
3 gigs of ddr3 ram
500 gig harddrive
windows vist home premium 32 bit
video chip 4 seires integrated graphics controller
is that what you were askin?
could you tell me how to find out the psu
 
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