I would be better if you could post the configuration of the PC.
Some Steps:-
1) First try each ram individually. Try running memtest86 on each one individually. If one of them doesn't work its faulty. If they work fine individually then go to the next step.
2) I presume you have 2 to 3 RAM slots. There is always a slot-0 then slot-1 then slot-2 and likewise...It is always prefered to start plugging modules from slot-0 and then slot-1. Slot-0 is generally the closest ram slot towards the CPU.
3) If nothing works then go to BIOS setttings and look at the RAM timings. Select auto mode if its available for CAS, RAS, TRAS, RP, tRCD, etc. or higher values from the current selected values. (Note: higher the value more is the delay but will be more stable)
4) Some motherboards may not be able to run two double sided memoriesat full speed.
(double sided is nothing but a mem module having RAM chips on both sides of the module)