Jayeff is right, disconnect everything that is not needed to boot in BIOS. Disconnect the hard drives(s), optical drive(s), floppy drive(s). If the motherboard has a built in video remove any other video adapters. Keep 2 sticks of memory and remove the other sticks. Connect the monitor and power up the system and be patient for a few minutes.
If you hear beeping or if you see indicator lights (power, disk, wireless, etc) blinking in a pattern then see if you can look up the error on the motherboard manufacturer's support page or user manual.
If you get the BIOS screen to start then check the settings. At this point add one device at a time, add the rest of the memory first and if it still boots then add one drive at a time. It is a slow process but if you make one change and the computer fails to boot, then you can undo the change and try something else. This will isolate what works and what doesn't.