Removing an old Xbox and PC from my Microsoft account

Removing it from the list doesn't sign it out. If you can still get to the old PC, sign out of your Microsoft account in Settings > Accounts there too, or it just re-appears the next time it syncs. For the Xbox you've sold or lost, change your Microsoft account password as well. That logs it out for good, removing it from the devices page won't on its own.

sign in to your Microsoft account page in a browser and go to Devices. Each one has a menu with Remove device, takes a few seconds. For the Xbox, if it's still set as your home Xbox, change that from whatever console you use now, otherwise the old one keeps sharing your games with whoever has it. And if you've actually sold it, factory reset it too, removing it from the account doesn't wipe it.