Question : Problem: Trying to fix a PC.... should I just give up?

I have some elderly neighbors that I'm trying to help out.  They had a PC die on them.  

It was put together by a local clone shop.  It had an Asus A7N8X-X and an Athlon XP 2700+.

The thing just would not power up.  I tried it with a known good power supply and it still would not power up (the LED on the board would light up when power was connected).  I figured it was the motherboard so I got a A7N8X-LA off of ebay.  It's the same motherboard except smaller (same chipset and the like).  I plug the new board into the known good power supply and get no activity.  At this point, I figured that it was a bad CPU.  I borrowed a XP 2800+ from a friend with no positive results.  I tried the power switch jumper on all the control panel pins just in case, but still no love.  I thought there might be something wrong with the power switch, so I salvaged one out of another case with the same results.  

So basically, I have a new board, a working power supply, and a different CPU and can not get so much as a spinning fan.  I'm about ready to give up.  Anyone have any ideas before I tell them to bag it?

Thanks!

Answer : Problem: Trying to fix a PC.... should I just give up?

The fan spins when +12v is available to it.
It doesn't care is the CPU or the rest of the Mobo are okay.
If the CPU or Mobo are bad that can stop the fan by way of protective features turning the PSU back off but if that's the case you should at least see the fan 'kick' and then stop.

If there is no fan spin at all then you aren't getting +12v 'to' or 'thru' the motherboard at all.
Usual causes are bad power supply or shorted motherboard caps.
Unusual causes are +12v shorts in drives, video card, or motherboard MOSFETs,,, a burnt PSU to mobo connector,,, or a short [usually mobo] to the chassis.
There can also be grounds in other voltages that keep the PSU from coming on.

At this point I would borrow a known good PSU and see if it will start.

Also try with minimal system.
Disconnect everything from power and motherboard - except:
Keyboard, one RAM module, what's needed for video, CPU.
You should be able to get into the BIOS or to a 'no OS' type error if you have removed the problem device.
It is advisable to do this on a piece of cardboard on a table [build system without the case] to eliminate the possibility of chassis grounds causing the problem.
.