So the BIOS works, the HDD spins, a few LEDs light, you can access the BIOS? You can move the mouse in the BIOS screen? Does the BIOS recognize all RAM modules, HDD, CPU, ... The BIOS has a self diagnostic routine. Run it, maybe it reports the failure. Try to remove all RAM modules, install only one and try to boot, try both slots, then try the other module in both slots.