The return of the mouse problem

Several days ago I posted about my mouse pointer working OK at startup then gradually getting jumpy, failing to move, changing arrow to text cursor and then back to arrow, disappearing, hanging up and finally either diappearing or stopping completely. It cleared up for an hour or two after posting (must have scared it) then resumed the behavior. In addition the screen is now jumping back a half page or sometimes more while I am reading web pages. If I shut down and reboot (getting the disk check routine) it will again be ok for a few minutes or sometimes longer before starting again.

I am wondering from where the basic operational commands for the mouse originate. Are they contained in the motherboard code or in the OS code, or perhaps someplace else I don't even know about. I have no idea where to begin looking. The mouse is a MS optical wheel mouse. Of course with the screen jumping around as well as the mouse misbehaving it appears to be something more than the mouse software. I did reinstall XP and the mouse software. Any ideas?