windows 7 memory problems

Ok it's been a long time since I've last posted here but I have no where else to turn to. i was given an HP M8300F to try and fix for someone. the problem is that the computer is having memory problems, the best I can tell. It started with when the computer was turned on, it would get to the windows logo screen before the login screen and would just freeze there. If you got to that screen you were lucky, the rest of the time the computer wouldn't even start. the lights come on but the monitor doesn't recognize a signal. So I tried doing a complete reformat. Didn't work. Eventually I started getting the blue screen of death. It gave a different error message every time but the common denominator it had was something to do about the memory. So eventually I started pulling memory sticks one at a time to test if one was bad. the 1st stick I pulled got the computer to boot and install windows 7. worked great for like 2 days. I gave the computer back but then the next days it was acting up all over again. So now I'm trying to trouble shoot it again. I pulled another stick ad worked all last night. turned the computer back on this morning and it froze at the windows logo again. Tried re-installing windows, but it get to about 2 mins into it and i get the BSOD again. Now it primarily says MEMORY_MALFUNCTION. What the heck do I do now??? I can't imagine that all 4 memory sticks that were in it went bad.

The computer isn't overclocked, had/has 3gb memory (2X 1g and 2X 512)Other than that it's all stock. It also doesn't have the recover partition anymore and a disk was never burned.

PLEASE HELP! I'm stumped.