If you've got a battery in the mouse, I'd suspect an IR connection to the computer. That eliminates bad cables. The receiver that plugs into the computer, and/or the driver for the mouse would be the likely suspects. Check for dirty contacts. If there's reason to suspect the driver, the usual first approach is to use Device Manager (Start|Settings|Control Panel|Systems, on the Hardware tab click Device Manager). Delete the mouse, and then reboot and let Plug-N-Play reinstall it.
Another device that shares the Serial Port (and/or USB = Universal Serial Bus) with the mouse can also sometimes cause interference, so dirty keys or bad cable/connectors on you keyboard can (rarely) make the mouse twitch.
The most common cause is a low battery in the mouse, and just because you just took a battery out of the wrapper doesn't necessarily mean that it's "like new," even for a one-shot battery. If you're perhaps using rechargeable batteries new means "just came out of the charger" since most of them self-discharge about as fast when they're not plugged into something as when they are.