A couple of suggestions:
1. The wiimote is now picking up stray infrared light. Is the sun coming from a different direction? Have you turned on more lights? If you are using SmoothBoard, you can check to see if this is the problem by looking at the calibration assistant screen. There should be no IR dots when your pen is not being clicked. If the cursor is really jumping around and perhaps windows are starting up by themselves, this is probably the issue.
There are two fixes if this is the problem. Pull some curtains or , if using SmoothBoard software, turn down the wiimotes sensitivity in its control panel.
2. The pen is defective and has become permanently 'on'. This is less likely, but can be very simply checked by looking at the end of your pen through a digital camera. You should only see a light when you activate it.
Let us know how you get on.
Dave
3P Interact Ltd
www.3pinteract.co.nz