Other than holiday time, are the computers normally left plugged in with the mains on, and allowed to enter sleep mode or standby mode (soft off) when they're not being used?
If they are normally used this way, it's not uncommon for the standby supply in the PSU (Power Supply Unit) to develop bad electrolytic capacitors, and be unable to start up after complete power-down. (Of course the printer has its own power supply). While the system remains connected to the mains, the standby supply keeps running despite having some weak capacitors, so the computer can fire up again. To prove this for sure, the most straightforward way is to get the use of a known good PSU and try it in the systems, which I assume you would be able to do as you have already swapped some components between the systems.
If this is not the situation, what precisely did you mean when you said "checked the power source - nothing"? Checked which power source how, and what did nothing mean - no power, no problem, etc?
Edited by Platypus, 04 December 2008 - 05:38 AM.