I was too afraid to open the computer up after having it working, and desktop computers are heavy.

Really? Why are you looking into PCIE SSDs? Why aren't you trying to get an actual GPU?
Vacuum cleans computer. "Fan supposed to go on at 80c" uh, fan should always be on. It should turn on well before 80c.

* Throttles at 100 C to around 250MHz (unusably slow unless I switch to the terminal) ???
* If running at 100 C for too long, will shut down

??? What? My CPU is at 30c.
A cooler isn't the problem, it's probably full of cat hair because you have never cleaned it or taken care of it.

Well, as I said I did do a little maintainence and got the idle temperature down to 40C, and I can watch 720p60 YouTube without it overheating, so that should help somewhat.

It's possible the fan may have been running at a constant rate without increasing at 80C. It *used* to increase in volume at 80C, but that stopped working one day.

Not looking into an "actual" GPU because they're expensive these days, I don't tend to play particularly demanding games, and I don't want to spend too much on a specialized piece of hardware. The one notable game that failed to run on this desktop is Ikenfell, and I can run that game just fine on a laptop with lower specs.

That laptop I mentioned? Celeron CPU, 4GB RAM, 64GB eMMC (which is slow, particularly in comparison to the PCIe SSD in my other laptop), and no fan, as it runs just fine without it. (Note that the CPU is more recent than my desktop's i5-4670, so the integrated GPU is more modern, and hence supports more modern features.)

clearly dtgreen is just looking for attention
how hard is it to buy a new cooler? costs like $10 max maybe
and all maintenance wouldn't take more than 2 hours
still couldn't fix it, pathetic

first hit? those are the advertised ones , ofc they are expensive
search for : intel 4 pin 3.5-Inch Fan For Desktop PC Computer it is just 9$

This cooler is much cheaper and will work fine as long as you aren't overclocking. I have personally used it for several builds.

https://www.amazon.com/Thermaltake-Gravity-Intel-Cooler-CLP0556-D/dp/B00O4UIZ86/

Almost any aftermarket cooler "works fine" and is better than an intel stock cooler. A quick search didn't turn up any in-depth reviews. I'm sure that it's fine if you don't care about how quiet your CPU cooler is.

My pick: The Cooler Master Hyper 212 EVO is great value for money. Why try to save 20 dollars on a CPU cooler?

Funny story; 8 years ago - I moved my desktop, car-in-car out, I was really careful with it - the intel stock cooler on the Core2Duo just fell off! 2 of those plastic pins were broken.
I quality cooler might weigh close to 1 kg - the through-motherboard connection is very solid.

1. The cooler I posted the link to uses screws and a backplate, not the flimsy plastic pins/clips. I have firsthand experience with it and can speak about it knowledgably. I cannot say the same for the Hyper 212 EVO.

2. "Why try to save 20 dollars on a CPU cooler?" Because 20 dollars may not be a lot to you personally, but for some people that is their week's grocery budget or gasoline/bus fare to get to work budget. Not everyone has and extra 20 dollars lying around to blow on PC parts.

@dtgreene

My advice if you're replacing the CPU cooler is to do research first, and read the instructions before ordering. This is because the replacement cooler may not use the stock mounting system - and replacing the mounting system can be done in multiple ways - from just unscrewing and replacing something on the front, to installing a full new back plate behind the motherboard (which will involve removing the motherboard from your case.

Based on previous posts of yours I have seen where you comment on not having opened the case previously because the desktop is heavy and comments on how some people wouldn't have a desktop on the floor because it's hard to bend over, there's a possibility that you're not the strongest or most physically able person. If this is the case, you might want to get some help or a professional to replace the part for you.

Actually, now that I actually bothered to open the case and look inside, I feel more comfortable opening the case in the future, possibly to install that CPU cooler.

With that said, I am putting this particular purchase on hold until I notice the CPU throttling again. (Running at full 3.4GHz and it's not overheating when watching 720p60 YouTube videos. Haven't tried running games like Hollow Knight (which I would expect to be playable on this system now) on it.)

This does, however, get me thinking about possible upgrades, like a GPU (but I don't want to spend too much on it), or perhaps USB-C (is there a way I could add it that would be capable of powering a Raspberry Pi 4?). Then again, there's the question of when it just makes sense to build a new computer entirely, as this computer is still rather old (and no amount of maintainence will make it new).

Here is a good video about applying thermal paste as done by a professional.

What's the output of your system? Your power supply unit needs to provide enough amperes, and your mobo has to let them through (most likely should). As for adding the ports, that depends alot on what options your motherboard has available. Odds are, you're going to want it in the front, but it might end up being in the back. Frankly, i'd be looking at USB-C adapters if I were you. I recently got one at walmart for about 2 bucks, because all I have is USB-Micro devices.

However the question is also worth asking if your USB controller on your motherboard knows how to adjust resistance to avoid over-amping. If not, it'll likely be limited to somewhere betwee 500mA to 2A, which seems to be a no for the pi.