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I have a friend that uses, and I kid you not, an electric leaf blower for computers that are next level filthy. I personally would not suggest this because (1) the air flow might be strong enough to cause damage, and (2) the air is unfiltered to you could be blasting the motherboard with grit and dirt. On the other hand, it’s it is an old PC with little value and it wouldn’t be a big loss, I might give it a try. I’ve seen a PC that was full of roaches before - if there had been a leaf blower handy that day I would have put it to the test.
Realistically, I would say just remove as much as you can of the dirt by hand, then used canned air to clean up the remainder. You might want to do it outside, because there is probably going to be dust everywhere by the time you are done. You also might want to wear some safety glasses to keep it from getting in your eyes.