First remove the power cord from the back, disconnect everything, including mouse, keyboard etc..etc... now open the case and disconnect/unplug all the inner components..like fan connector, cpu, ram, front panel connector, usb, audio, PCI, etc..etc..
Refer this:
[If there's Add-On graphic card, use the On-board one.]
Note all the connectors/plugs and their respective corresponding male/female connectors/slots. Once you are done disconnecting, remove the Bios Battery. Leave it outside for few minutes. Now install the battery, cpu, connect the cpu fan, install the ram (if there are multiple ram sticks, install only one of them), connect the hard drive, the front panel (only the power switch). Now connect both the power connectors (see above diagram, i've circled the power female connectors with red circles, there are two connectors..).
This is the labelled diagram of your motherboard.
Now power on the Machine and see what happens, if it's ok.. Switch off the machine, reinstall the remaining connectors and components one by one and power on the machine everytime you connect/install a component. Don't ignore this, otherwise you'll skip the most important part and you'll not be able to find out the culprit.
After several attempts and reboots, you'll see which one is causing the problem and you have to get rid of the particular component.
Let me know the result.
Good luck.
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FixYa.