First thing to try is loosening the Screws that hold the Motherboard to the Case. They will only need a full turn. Reconnect the Power and turn it on. If it powers up shut it down and retighten the screws. They only need to be firm.
Carefully remove everything from the Motherboard. Remove the Motherboard and put it on an anti-static mat on a bench. Remove the Power Supply. Remove the Heatsink and give it a good clean. Remove the CPU and reseat it. Reinstall the Heatsink remembering to use CPU grease. Clean the golden edge of each memory stick with a soft rubber, remembering not to touch the golden edges. Check with one stick at a time, remembering to disconnect the power from the PC between checks. Connect a hard drive. If you have access to a PCI Video Card use it.
Reinsert the 20-24 pin power connector to the Motherboard. Connect a Keyboard and Mouse. Connect the Monitor. Connect the Power to the Power Supply. You can use a flat blade Screwdriver to short the Power pins on the Motherboard if you haven't got a spare switch. Turn it on and see what happens.
If that PSU is only rated at 230W replace it with someting around 450-500W and dont go cheap as they are inferior.