First check the power switch either hook up to the wrong connector or backwards on the motherboard or the switch may be bad. The light you describe sounds like it is the standby power indicator.
If this is not it, your motherboard may be grounded somewhere. Remove the board from the case, set it up on some foam, put the memory in and connect the power supply. Leave you hard drive disconnected. See if you can post. If so turn off and hook up your hard drive. See if it will boot. If so then there is a good chance that you shorted out the MB. This can happen by over tightening the mounting screws, or some metal shorting out somewhere between the MB and case.