memory - PC doesn't load properly when I upgrade from 2 x 512MB RAM to 2 x 2GB RAM

I recently upgraded my RAM as it was only 2 x 512MB DDR2 2.1V, 800 MHz.

I tried to change it to 2 x 2GB DDR2 1.8V, 800 MHz but the PC would not load properly. I now have one old and one new memory in place totalling 2.5GB RAM.

Why doesn't the PC work when both the 2GB sticks are put in?