Blue screens and freezing problems

My PC's been giving me a blue screen saying one of the below:


I originally had 2 1gig DDR2 sticks and replaced them when my computer kept freezing. When I replaced them with 2 2gig DDR2 sticks I would get one of the above 2 blue screens and noticed it would stop when only having a stick in one of the slots. Now I'm getting them again as well as the third blue screen. Any ideas? Thanks in advance.

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