Can you help me with something?

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I have some problems... I don't understand something. It happens almost everyday: When I'm on my computer, my mouse starts moving even if I don't move it. Let's say I'm reading something, and I'm not moving the mouse, but it starts to move itself! And the odd thing is that I don't think there's someone in my computer, because the mouse just goes up and when it reaches a corner of the screen it stops. It's doing that pretty often! Is there something wrong with my computer? Or is it actually someone from another computer that can control my computer? (It happened to me before).