Long story short... I was switching the RAM in my two Win 7 64bit desktops 4G<->8G (2 sticks ea) and found that the 8G sticks (2x4) only work in one configuration. We'll call them stick (a) and stick (b).
The system won't boot if the 4 Gig sticks a and b are in slot 1(a) and 2(b) but if I reverse them to 1(b) and 2(a) all works ok. It's the same on both computers.. both have ASUS MB's w/Core i 3/5 CPU's. RAM is Kingston DDR3 240 pin. They are in the correct pair of slots. Hope that's clear.
It was aggravating not only for having to switch back and forth but mostly to reset my all bios settings which reverted to default each time it didn't boot.