laptop - New RAM sticks don't work with old ones

I bought and placed an additional 2GB of RAM in my notebook. I went to the setup screen, and appears there, I added the memory (2GB + 2GB = 4GB).

When I continued to boot the notebook, the screen indicated that Windows 7 was loading, but before the operating system finished loading, a blue screen showedd, which suggested to me that the new memory was not working (somehow).

I then used only the new RAM modules in my PC and to my surprise it worked again, which leads me to believe that the conflict was between the different memory sticks.

How can I be 100% sure about that?

Edit 1: The blue screen message is:

A problem has been detected and Windwos has beed shutdown ...

IRQL_NOT_LESS_OR_EQUAL

BLABLABLA
Technical Information:

STOP: 0x00000000A (0x0000000000000000, 0x0000000000000002, xFFFFF8000C0CA869)

Edit 2:

My Windows is 64 bits, this influences into something?