My Macbook Pro crashes every time I change my desktop
If your Mac still works, I would connect a USB drive (or other) and make sure Time Machine gets a full backup before anything else goes wrong.

At that point with your files safe, you can use OS X Recovery to run Disk Utility. The first link has some very general items in case this is the first time you've heard of Recovery or Disk Utility.

As you troubleshoot this, you can either ask a follow on question or edit the question if you can rule out some things you have tried or need more specific help using / deciding which step of these tools and guides to take next.