PC randomly freezes

So i've got my pc more than a month ago and still can't get it to run properly.
My pc:

mobo - asrock n7ad-sli
processor - intel Q9550
4gb dual channel ddr 800mhz memory
1gb palit gts 250 graphic card
500gb samsung hdd
750w corsair tx series psu.

so just as article name says, pc keeps randomly freezing. First of all I thought it could be caused by psu (originally had 600w cooling something £20.00 cheapou) but unfortunately this upgrade did not sort my issues.

I try to run random video tests but it never goes through. Thought it's video card but now pc freezes even while writing this message (my third try)

please advise of possible solutions out of this nonsence

thanks,
Simon

Hi Youngster, there are two different types of freezes happening:
1 - pc freezes to a black screen
1.1 - and either restarts
1.2 - or needs to be reseted manually
1.3 - or gives me a blue screen with nvlddmkm.sys error
2 - pc freezes to a screenshot that is currently on
2.1 - and sometimes restarts
2.2 - but most times have to be reseted manually.

I have reinstalled OS maybe 20 times and on Vista it was working a bit more stable. However still none of the tests were passing properly. Now I have windows 7, and nothing changed. any thoughts?