Please give us a little more info in regards to the hardware you have in your PC. Namely the motherboard & RAM you have, but other hardware specs too if you know them. If you aren't sure, download CPU-Z and post Screen Shots here. A general rule of thumb is to never mix n' match RAM. Sometimes it'll play nice and sometimes you'll have nothing but problems.
A little more info and we'll have you back up & running in no time.