PC restarts at random time

my computer keeps restarting at random moments....tried changing the PSU nothing happens , reformatting the hard disk the same thing...cant even run the memtest coz keeps on restarting...what the hell is the problem with this one? i have asus p5s-mx se mobo, 1 gb memory, and 160 hd...
 
For clarification, heve you tried memtest or you haven't tried it because of the constant restarts?
 
I had the same issue with corrupted memory. Is your memory in one stick or 2 512 sticks?
 
only one...ive solve the problem....it not the memory but a damage HD...i just remove that one then its working fine now..
 
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