I have a recurring problem with my mouse speed halving.
What happens is after first booting, my mouse behaves exactly as setup. Then at indeterminate intervals and following no specific event, there is a "Bloop Bloop" sound following which my mouse speed/sensitivity halves. Like walking through treacle. Subsequently in an equally random way, the mouse returns to normal. Sometimes (but not always) this return to normality is accompanied by another "Bloop Bloop" sound.
Because of a Microsoft log jam, I formatted a couple of months ago and thought to myself that at least that Bloop Bloop will dissapear, but not so. It is still there.
Running Windows XP Pro fully updated with a Microsoft IntelliMouse Explorer USB wired not cordless.
Sounds like a designed-in software feature that's being activated. Have a good look through the mouse driver options (i.e. Control Panel/Mouse). Probably there is a checkbox labelled something like "Enable auto precision" that needs to be unticked.
After exploring every thing (and more) I have now had 24 hours without the symptoms returning. The fix (or fudge) was to remove the USB Mouse plus software and reintroduce it via a PS2 adaptor. No further Bloops since. No reasonable clue as to why though.