Go into windows with one ram stick, right click on my comp -> go advanced tab -> click on startup and recovery ->uncheck the box that says restart on error. Now start up with the 2 sticks and see what it says, my money says it give you an evil blue screen saying your ram is rooted. So now when you start up again go into bios instead and put your ram timings down down down, (if running quality ram) go to 2.5,3,3,7. Tell me what happens when you do this. ALso try running memtest (make the disk in windows with the single ram stick obviously) with the new timings i stated and see if it passes.
Also make sure you have all overlcocks off and set speeds to stock, including voltages