The quintessential, alchymical Wintel black box and I don't have a real clue to lead us through the labyrinth (fortunately others do). This linked text [taken from a comment by *jake* appended to a recent El Reg article *FBI 'planted backdoor' in OpenBSD*]
ACM Classic: Reflections on Trusting Trust can be followed by a general reader (me) and should scare everyone.
My mind-set is that everything is hardware and that software is maya, illusion. I had assumed that you were using a PS/2 keyboard. Clearly the external mouse is fit and well ... I was considering the path taken by a notional differential voltage as it propagated from the USB port across the motherboard to some unknown final destination there to be read as a Mouse_Event_Multi or whatever the grownups call it. If you could reproduce the failure along a different signal path from a keyboard controller via PS/2 then you could be almost certain that the problem lay within the Windows environment. The default keyboard can be used (within limits) to control both the mouse pointer and the buttons ... I was suggesting you gave it a bash when next the error condition was encountered. I have seen apparent software (i.e. hardware) faults that only manifest in certain mysterious, albeit reproducible, switching states. I have not checked the block diagram for your specific motherboard but they are all much of a muchness.
However, the one thing that Microsoft always got right (both hardware and software) was the mouse. I've tested various operating systems and rejected some within the first minute simply on the basis of the *feel* of the mouse. If you want to mess up someone's day then give them a crabby mouse to work with ... so empathy.
As an aside, I read in one of the PC comix that the Intel 65 nm process had been less than successful (now they tell us) ... mine's the beat-up Zilog (I was happier then). I had to use my fingers today to work out which was larger, 2/3 or 3/5, so that's probably Why Johnny Can't Add.
PS: I don't deserve hitting ... soy una vida en ser. Malcolm always meant well.