Can someone else add POP and IMAP to my computer?
To even ask that question suggests that you don’t know much about what they are, or how things are “added” to a computer. It’s not really clear what you’re asking. You might mean if they don’t have physical access to it. But if it is hacked into then of course they can. Any software or programs can be installed. And a previous owner can put something on there and you’ll have it.
I can still get email messages as before, but I’m not able to
completely log out of some websites
That’s like saying I can still eat sausages but i’m not able to kick a football. You have 2 problems, not one problem, or if it is one problem, it’s one enormous problem(a problem so enormous it might even cause you to write nonsense questions).
Microsoft is one
What does that even mean. Is that a theological doctrine. “Hear O World, Microsoft is our deity, Microsoft is one” Sueteronomy/Billteronomy 6:4
Someone has installed a local disk on my computer that I’m not allowed
to open. What is this? ….I think I managed to remove the disk but
I’m not sure.
That’s like saying that somebody came round to put a TV in your house, and you think you managed to remove it but you’re not sure.