1) Why was the motherboard replaced?
2) If this is in reference to an HP Pavilion Media Center M1070n, it uses an Intel Pentium 4 Model 520 (2.8GHz) processor.
Did you make sure you plugged the 4-pin ATX +12 Volt power cable, into the motherboard?
This is power for a Pentium 4 processor, and on up.
This is HP Support, and the Motherboard Specifications page, for the HP Pavilion Media Center M1070N,http://h10025.www1.hp.com/ewfrf/wc/document?docname=c00208497&tmp_task=prodinfoCategory&lc=en&dlc=en&cc=us&lang=en&product=426501
Scrolling down the page is an illustration, and a photo of the motherboard.
Below the Processor socket is the Northbridge chip.
(The Northbridge chip, and the Southbridge chip are the motherboard chipset)
Intel i915P MCH
In the motherboard photo, and installed in your computer, the Northbridge chip has an aluminum heatsink sitting on top of it. (Has fins on top of the heatsink)
Going across to the Left, from the top left corner of the heatsink, is a square connector with 4 socket holes.
The female connector on the motherboard, for the 4-pin ATX +12 Volt power cable.
4-pin ATX +12 Volt power cable,http://www.playtool.com/pages/psuconnectors/connectors.html#atx12v4
Two Yellow 12 Volt wires, two Black Ground wires.
Let's start with this, and go from there.