Your motherboard manual(s) is your best friend.

Rip everything out of both systems except mobo, processor, RAM and Display card. Double, triple and fourple check all the connections, especially the front panel connectors. Get the systems POSTing with that simple configuration first, then install and connect other components one by one.

Things as simple as a card or RAM module not seated correctly, or a floppy drive power connector that's accidently 'missed a pin' can stop a system from booting. If everything is connected up from the outset, it can be almost impossible to track down where the problem is.

Slow and steady, step-by-step is the way to go.

Also make sure nothing is grounded if all else fails take everything out of the computer and put it all back in.