bsod 5 times till now 6.1.7601.2.1.0.256.1 locale id 1033

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brought a second hand laptop lenovo g570 running win7 ultimate 64bit
intel 2nd gen i3 2310m @2.10ghz
4gb ram
seagate 320gb hdd
done update by window update and intel driver update tool
bsod occurred first time when coming out of sleep mode and second time after subsequent restart
it happened exactly same again after 2 days.
i have already ran disk check, driver verifier which lead to another bsod, kespersky, malwarebyte, esat, rootkit remover etc.
i've also done all basic test on seagate tool which hdd passed
will do memtest86 tonight.
what is the cause of bsod please help if it a hardware related issue i'll replace the machine.
can you point out exact reason of bsod or its trial and error?
thank you