I am up to the checkdisk and clean boot part and so far, nothing has worked. It's slow going because I have to work for a couple of hours before I can be reasonably confident that the problem is or is not gone.
By the way, on totally unrelated note, do you know why my cursor keeps disappearing while on this site and what, if anything, can be done about it? I don't mean my mouse pointer (which has the opposite problem, it's supposed to vanish while I'm typing, but on this site it doesn't), I mean the cursor that marks the place I'm typing.
@DominicP: I blew out the sensor hole with compressed air after I read your post. No noticeable difference. And even if that had fixed the jerky movement, how could it possibly have fixed the ignoring of my clicks? (BTW, there is no obvious way to open up the mouse and look inside.)