Won't boot anymore...
So my PC which has been booting fine for years, has now just stopped getting any power, no fans, lights or anything. I have verified that the PSU works fine with other systems, have tried various combinations of RAM (1 installed, 2 installed, both slots etc) have stripped it down to bare essentials and sill nothing.
I need the full system specs for all the computers you tried the PSU in.
When a computer won't come on after years of usage, I would say that its about 99% likely it is a PSU problem. They wear out many times faster than other parts, especially if they are top mounted.
I think that your PSU isn't up to the task of powering the main computer anymore and that it is up to the task of powering your other systems which I am guessing have less steep power requirements.
Without more information, that is my best stab at it.
That wasn't really the full system specs that I was after, but I am going to assume you mean the 120w is a Pico PSU which doesn't fall within specs for 12v power delivery even under the lightest loads (+/- 10% doesn't cut it).
If anything I lean more towards the PSU as being the problem than I did before.
All electrical components get worse the longer they are used. It doesn't matter what they are.
Every thing regardless whether it is a PSU, a TV, a refrigerator, a car or whatever else that has an electrical system in it performs some amount worse every year than the year before it.
Eventually, those things perform poorly enough that you want to replace it.
PSUs, however, are almost singular in how fast they break down. Every other part in a computer tends to have a much longer average life expectancy than the average life expectancy of the PSU.
Even the best made PSUs tend to break before, say, the motherboard, the processor, or the video card.
This is just a fact of life with computers, not even Bill Gates can avoid it.
Many people completely replace their PCs before their PSUs degrade to the point of failure so they can avoid the PSU failure on that basis, but rest assured if you turn the computer on and use it and off again every day that one day it won't come back on again and it will 95% likely be the PSU that failed.
Feel free to look through the posts on here and seek out threads that say "worked for 3 years but now it won't come on" and the first response will almost always be "new PSU" and they will get one and it will pretty much always be fixed.
Many people on their own will get new motherboards and new processors and things before they stop by here and that almost never fixes the problem.
This sort of thing is just expected to occur.