How can I find out what my computer's doing and is there a way to stop it?

For most of the past 5 days, it's been busy doing SOMETHING but I have NO idea what!! Any time I've got it on, it's clattering away non-stop the entire time it's on and it's taking me FOREVER to be able to do much of anything else on it!!
How can I stop this bullshit and regain access to my computer?

How can I find out what my computer's doing and is there a way to stop it?
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