Cannot Boot into Windows After RAM Upgrade

According to my manual and Dell's website, I can upgrade my RAM to up to 4GB. I purchased compatible memory from G.Skill and installed it today. After upgrading, my Inspiron 1420 could not boot into Windows. I did Startup Repair, Dell Diagnostics, checked the BIOS (revision A10), and everything else I could think of or research. I reinstalled one of the 1GB modules along with one of the new 2GB modules and the computer booted fine, showing 3GB of installed memory. I removed the 2GB module and replaced it with the second, new 2GB module I purchased. Again, the computer booted fine. So, both modules are good. It's only when I install both of the 2GB modules together that I cannot boot. Can anyone help me?