Do you have siblings? Because I got another mouse and was able to pair it up with the same computer and mess my brother's mouse movements - it was great fun. It might not be software related, but just harmless fun
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But in the more likely case that it is malware related, do what dormantreign said. If it's via spyware then he has to be controlling over the internet. Unplugging it should determine whether you have spyware or not.
Also, change mouses before installing a new copy of windows. That will also narrow down the problem - if it's fixed, it's a hardware problem. If it's still there, then it's software.