I recently got a new laptop. They're both Windows, so I used the same Microsoft account to log into both. When I turned my laptop on, it had the same background as my desktop, so I changed it, only to go back to my desktop and see that it changed to my laptop's new background. How can I keep the backgrounds separate?
To disable the synchronization of the certain settings to computers with your Microsoft account (on Windows 8):
The one that controls the desktop background is Appearance, the second one under the Personalization settings header. If you don't want any settings synced, disable the very top setting, Sync your settings on this PC.
On Windows 10:
The one that controls the desktop background is Theme here (source).
Changes to these settings take effect immediately.