Thanks for the advice guys. I have now discovered what's causing the problem - although not WHY it's causing a problem. A week or so ago I installed a 7-in-1 card reader on my PC and it appears to be this that's causing the autoplay search window to pop-up. If I unplug the reader the problem stops.
The strange thing is though, is that this happens even when there are no cards in the reader!? If I open My Computer when the card reader is plugged in, the removable drives that exist for each card type (Smart Media, Compact Flash etc), appear and then disappear again while I sit and watch!!! For example: the Smart Media removable drive appears (with a drive letter), then the Compact Flash removable drive appears, then the Memory Stick removable drive appears etc. Then weird things start happening - one by one each drive will then disappear and then reappear again in sequence!! All this happens without me touching the card reader or it's USB cable and without any cards inserted. Other than this the reader works fine as soon as I insert a card of any type.
I have to admit that I've never seen anything like this and it's got me completely baffled. Anyone got any ideas? Thanks.
Neil