Hey, from your post it sounds like your looking for a motherboard and cannot find one, sounds like u have tested the PSU, and I'm not certain what switch is good, but if u can provide me with the motherboard number I'm sure either I or someone here can find u a mobo for this PC, alternative, I'm not certain that this PC is actually worth getting another mobo for it
http://m.ebay.com/itm/eMachine-ET1831-2GB-500GB-Intel-Celeron-CPU-450-2-20GHz-7-DVD-Burner-Memory-Ca-/282006558969?nav=SEARCH
Here is a similar PC and is going for 110$, this might be the route you take
http://m.ebay.com/itm/MCP73VT-PM-eMachines-ET1810-ET1830-ET1831-Motherboard-w-cpu-and-ram-/222077343625?nav=SEARCH
Here is a mobo I think it might be the right one but I cannot confirm without the mobo number, but see the price for a full one of these is going for around the same price as the mobo itself, but let us know if u need anything else