Gaurav,
Thanks for the reply.
However:
(1) the problem occurs on a computer without a touchpad;
(2) the problem can be prevented.
I know 2 to be the case because just recently I have written a program that prevents it. It's an Autohotkey script. I've put it - a compiled executable file, to be run on boot, together with the source code, which if one has Autohotkey one can compile or interpret - here, on box.net. I haven't tested the program exhaustively but it seems to work well. (Also: possibly the compiled file won't work on 32-bit systems . .)
EDIT: If I, a self-taught amateur programmer who isn't really any good, can do this in ten minutes, there's no reason at all why ClassicShell can't do it.
Further edit: 'There must be one specific program causing this' - why? This focus-stealing sort of problem is one that has beset Windows users for years. I fear that what you have said is false.