I've got a Acer Aspire

Question

I've got a Acer Aspire 5612ZWLMi with Vista Home Premium, supplied with 1GB DDR2 RAM. I upgraded to 2GB RAM (1 module) last year with no problems. I have just bought another 2GB module to increase up to it's 4GB max.The problem is with both memory modules in place together it does not boot up, no post just black screen. Either module works alone in either slot, just not together.Any suggestions please?Regards Steve