This means I'm now on the backup mouse, which is a MS IntelliMouse Explorer (the v4.0 one with fancy sideways scrolling). The one with the weird scroll wheel which doesn't go click, and appears to have a maximum scrolling speed which is below that of the old one. Attempting to scroll faster than that results in it going slower.
And the scroll wheel only works when the mouse is on the mat. Pick it up so the led on the bottom starts flashing, and good luck if you can scroll more than one line per revolution. My best guess is that for some unknown reason the led for the scroller (the scroll wheel usually uses an opto-encoder) is powered off the same line as the led for the optical sensor. And to make the fancy sideways-scrolling work, you have to install a more recent driver, which a) no longer scrolls in VB6
>click< >click< >clickclickclick< >CLICK<
Oh, and the left button is a bit tempermental. Try to double-click the control box to close a window, and windows pauses for a moment before popping up the system menu. A similar thing happens when double-clicking on icons, or single-clicking links, or clicking anything. Not impressed. Time to go dig out the soldering iron, and shorten my mouse cable by another couple of inches.