Bah. Looks like it is a hardware problem; I've just tried using the Windows 2003 box plugged into the same KVM -- opening a web page with plenty of text and randomly single-clicking on words on it causes about 1 in 10 clicks to be misinterpreted and to select the word (i.e. seen as a double-click).

I'm loath to replace the KVM -- it was quite expensive, and quite hard to get hold of (I had to get Hugo to bring it back from the US for me). I'll see if I can try a replacement mouse in the next day or so...