Just assembled a bare bones computer
Starts and the shuts down

Only have the power supply and the hard disk attached to the computer
still get shut down before computer even posts

Some help of specs my help (aka mobo, cpu, ram). Make sure that everything is seated correctly, reinsert everything. Make sure that the mobo isnt touching the case at all. Also, make sure that the heatsink is properly attached and thermal compound is applied correctly. Do a search on another computer to find instructions, usually found on your manufacturer's web site.

Only use the mobo, cpu, heatsink, ram, videocard(if necessary).

The problem i had was not any of the above but the fact the leads from the case were improperly connected. One lead from power+ was on the wrong pin. Make sure you scutinize your manual to make sure they are properly attached. Also make sure the CMOS jumper on your mobo isnt activated. Hold down the button on your case for at least 10 sec.

Post back your results

Good advice above.

Since iit does not post, does the mobo have a "clear" jumper? Did it ship set to "clear"?