I would say that somewhere in the replies here you likely have your answer. Either its a bad stick of ram, or a former good stick of ram gone bad, or its a program causing the problem.
Depending on your os you can stop some programs from starting at bootup and there is one more possibility, i would say its likely 3rd in line, but there are programs that install themselves without our approval, usually via the net, and they cause probs sometimes. To check that aspect out you can google and download "hijack this" and/or adaware and/or spybot search and destroy, all have free versions. Hijack this is the most complicated of the bunch.
But, if you have tried the first two suggestions and still have the prob, you could try the third. YOu can even cut and paste the results here and get help to deciper the results as the people around here are very very good at that, i know as they have helped me and many others with that.