If it is mostly dust, you might be surprised how much you can get out with an air compressor!

I have a couple, a bigger one to run air tools, and a little "tankless" one which I often use when working on/cleaning
out smaller things. With old really-dusty stuff it helps a lot!

Couple things to note:

If it's as dusty as you describe... take it outside! lots of dust will blow "everywhere"

Use a bit of plastic/wood to block the fan from turning... you can really overrev them if you're not careful.

Dave (If you don't have a compresser, perhaps a friend does - most garages do well)

Will canned air work as well?

I guess I can put it out on the balcony when it's not raining to see if that does anything

Canned air will help, but a compressor will be able to blow it all clean without necessarily having to open the thing up.

DaveDDS wrote on 2025-01-27, 00:22:

Use a bit of plastic/wood to block the fan from turning... you can really overrev them if you're not careful.

To be clear, the problem with spinning the fan is that it acts as a generator which can subject electronics to higher than design voltages. Other than that, you could rev the fan to the point it flies apart and the only thing you'll have to replace is the fan itself (assuming the shrapnel isn't too egregious).

Can you open just the top cover of the PSU and show us some pictures of its insides?
Don't worry, nothing will "jump out" at you. Just obviously make sure the PSU is unplugged from the wall before trying to open it. Apart from that, most old ATX PSUs are pretty safe to open, as they typically discharge their high-voltage primary caps almost instantly (or within 10 seconds about as bad as I've seen.)

The reason I ask for pictures is because some old PSUs are worthy of saving, and some aren't.
In your case, you have a Pentium 3 rig, which isn't anything too power hungry and will usually run even on the cheesiest and crappiest of PSUs... but still, I like to see what I'm dealing with every time. It will also reveal if the PSU may have developed bad capacitors.

Now, if the PSU is worthy of saving, a can of compressed air should be able to remove most of the dirt inside it when you have its top off, unless...
... unless, it's really one of those PSUs' that's been in service for so long that it's literally caked in dust. I got a few like that last December and had no option but to completely take them apart and wash everything. YES, you CAN wash the electronic parts of a PSU with water. Just have to let them dry properly (a few days under sunny breezy weather, at least) before attempting to give it power. Only thing that shouldn't be washed with water is the fan - that, I clean with a wet wipe when it's out of the PSU.