Computer keeps rebooting. I recently decided to upgrade the RAM in my computer from 512mb to 2gb

Question

I recently decided to upgrade the RAM in my computer from 512mb to 2gb which is the maximum my motherboard will hold. Well, I can run 1gb fine and I can run 1gb stick and a 512mb stick fine as well. However, whenever I insert a second 1gb stick it will run for about 15 minutes then reboot and it will show a message that states "Windows has recovered from a critical error."

Some have told me it could be either power or cooling problem so I went ahead and bought a new case that has seven 120mm fans cooling it and a new 1000W power supply and nothing has changed. Could it be windows or something else?