OK, after a lot of experimenting I nailed down a way to repeat this problem which had occurred a couple of times. Once again it shows up because my PC is geriatric (i.e more than three years old).

I am a bit ambidextrous so I like to paint using the tablet pen in my left hand, then use the mouse with my right hand from time to time to change settings, etc.

I have discovered that if I use a slower tool like water colour and I lift the tablet pen off the tablet before the stroke has finished rendering, the cursor goes a bit wrong:

I see the tool cursor stuck at the point where the pen left the tablet, and it doesn't move if I move the mouse. Then when the (invisible) pointer gets over a menu, panel or other screen widget, the appropriate pointer appears in the right place (and vanishes again if I move off that widget). The problem persists (including minimising and maximising and even starting a new painting) until I bring the tablet pen back into range of the tablet, at which point normal pointer operation resumes.

Everything works fine if the stroke is finished drawing when I lift the pen off the tablet.

I tried installing the latest tablet drivers from Wacom but that didn't help. It's only a minor glitch but I thought you might like to know.

Windows XP Pro / Wacom Intuos 4 / Artrage 3.0.4
AMD Sempron 2800+ / 1.18 GB Ram