What is going on with my computer?

What is going on with my computer?
I had this problem once before when I was running low on memory, and it resolved after deleting several files, but now it doesn't seem to be working.

It tells me I still have over 600 GBs of space, so I don't know why it's doing this. I've been deleting files and doing updates, but the computer still thinks it's low on memory space.

Now it's reached the point that it won't even let me delete files to make more space. It gives me that error message when I try to put a file in the recycling bin.
It won't even let me open browsers/programs.

How can I fix this problem permamately? I have Windows XP and a Dell PC, that's almost a decade old.

We've been having issues with the WiFi lately, getting a security error. I'm wondering if something the tech people did, caused this.
But I think it's all the media files I have on there. But I don't understand why deleting files isn't fixing the problem.
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I ran an Error Check/Repair, and it seems to have fixed it. *Knock on wood
What is going on with my computer?
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