What kind of CPU and speed do you have now?? From my experience here's what could be wrong:
1)New RAM is incompatible with your old stuff
2)You could be maxing out the board and it doesnt know how to handle it.
3)Bad DIMM
It could be one or a combo of things, did you ground yourself(touch an unpainted part of the case) before handling the RAM? Static electricity built up in your body could have zapped the RAM thus pretty much destroying it.
Post your PC specs such as CPU type and speed, and what kind of motherboard (Tyan, Asus, Microstar, etc.) and the model. And the current amount of memory in your PC. Some people will tell you that all DIMM memory is compatible. That is partially true, if you have the same speed and timing throughout the memory bank. If you bought PC133 or PC150, it is backwards compatible to PC100 and PC66, but I have seen problems where the newer memory does not work at all in older systems. Post the specs and we'll go from there.
