First of all, make sure you are running Windows 64 bits and not 32 bits. If not, then there is your answer.
If you then also make sure the first RAM is int he slot closest to the CPU and the second ram is in the third slot.
Next, reset bios settings to default and see if that fixed it. If not, have a look in the RAM configuration in your bios. Since this is the technical stuff, I would advise you not to play around with it much but still, try to look at the options available as well as looking up on google what certain options mean and set up the configuration accordingly.
Now, if that didn't fix it, resort to checking your OS