I agree, you do not need a new mouse. But this is typically a TweakUI PowerToy (or similar registry tweaker such as X-Setup) setting for "X-Mouse" called "activation follows mouse".
That said, you can test/verify this by opening multiple windows and move your mouse back and forth over them. If the focus changes, then this X-Mouse setting has been set.
If this ONLY occurs with pop-up windows, then I would be concerned with malware infestation and would recommend you run complete scans of your system - see
Cleaning Out Malware.