couple things to try. Reseat memory, CPU, and vid card also try with ONLY the HDD, CPU, mem and vid card. No cdrom, floppy, keyboard, or anything. See what happens, yeah you'll get a keyboard error, but we just want to get a screen. If nothing, you may be grounding out the MBD to the case. So, try removing the MBD with the above bare system specs and place it on something that is not ESD conductive and see if it will run with the MBD COMPLETELY out of the case. If it runs like that, then its grounding somewhere in the case, maybe a screw or something. Should have a sheath that covers the metal screw with something made of plastic or another non/less conductive substance.
EDIT: Also, when reseating check things like the pins and such, like the CPU for bent pins. Also, make sure they are firmly seated, memory for example should click and lock into place. Check for excess/lack of thermal compound on the CPU.
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