Hey.
I used to run various older builds that my grandpa would bring in from work. They were typically from the mid 2000s. Y'know, home family type PCs. They usually had bizarre, nonstandard ports and sizes, and could very rarely have any of the parts swapped out except for stuff like the CPU, GPU, RAM, etc. etc.
I decided to install linux on them. Mainly because it would be easier that way, instead of trying to struggle with making a modern install of windows run on it. These would be very bad dual cores that, even in their day, sucked. I loved installing linux on them more over the bland windows installation process as well. It's crazy how well youtube would run on them, and how light on resources linux was. Even turned laptops that would most likely never see the light of day within the past few years into a respectable machine that could conquer simpler tasks like light web browsing, word processing, checking emails, occasional videos, etc. etc.
I don't really do this anymore, but I do still retain most of the knowledge of what I would do in order to breathe new life into these old machines. Most of the time, all you need to do is upgrade the ram to a respectable amount and install linux. That's basically the bare minimum. But if you wanted to go all out, then this becomes a little more complicated.
First of all, here's a no-brainer: If there's a much better and more efficient yet affordable CPU you can get your hands on, then buy it and swap it out. The CPU is the main thing you need to work with above all. It determines the absolute performance of your PC in nearly everything it does. The RAM is your next step. Without sufficient ram you will have a bad time. Make sure there is enough so that your PC doesn't start to choke and stutter from a lack of it, but don't go overboard. And then comes the GPU. The GPU is a necessity if you want to work with anything graphically intensive. Make sure you get a GPU that can actually work with the CPU within the machine. You don't want to waste money only to have a bottleneck. You also don't want to get an underpowered GPU to match your CPU, unless you determine a much cheaper and less powerful card to suffice.
You should also make sure the PSU is compatible wattage wise with the new upgrades. As well as making sure it has the appropriate cables for your CPU, GPU, and Storage. Get the listed down wattages of your CPU and GPU, and make sure your PSU has a little over the required wattage combined that is demanded from both your CPU and GPU.
Also, these things will most likely have old HDDs in them. Get an SSD. A fairly good one. And put the OS onto it. You will not regret it. The boot times are unmatched. And it will feel like brand new if you do so.
And get a better HDD. The older ones seriously suck. But depending on what you're doing, then you're gonna want some additional storage. The 250GB SSD and 1TB HDD combo never fails. And the tech is cheaper due to innovations and breakthroughs in tech, so you won't have an issue with affording this.
You might also want to make sure that the interior of your case is clean from dust and other contaminants. As well as replace any fans that are defective or not powerful enough to provide proper ventilation. Fan replacement is cheap, but make sure you can actually plug it in and mount it, of course.
And also make sure that the thermal paste is replaced. If you decide on not replacing the CPU or GPU, then you could, at the very least, swap out the old and crusty thermal paste for a fresh new layer. Cleaning the plate is easy, and taking off the heatsink isn't as hard as it seems. And good thermal paste doesn't cost much.
After changing the HDD, PSU, GPU (If there was one!), RAM, TP, etc. etc. and installing a lightweight modern distro onto it. Then it will be a beast for what it is. The transformation would be near unbelievable. And you will feel a sense of accomplishment in achieving the revival of an old system. The ultimate form of recycling.
Anyways, those are my tips on how to revive old PCs. If you need any help, just @ me on here or reply. I've got a lot of experience working with these kinds of PCs. From enterprise to P.O.S. systems, you name it.