I don't think a dirty mouse is the cause of this. Actually, a dirty mouse would cause just the opposite effect...the mouse doesn't move when it is supposed to. When the cursor suddenly leaps from one side of the screen to the other (with no trail in-between), I believe it is a memory conflict. It happens to me when I have too many programs open, or haven't rebooted in a long while. Some program is intruding on the bit of memory reserved for mouse operations, and as such your pointer location gets forgotten / reset.
Run fewer programs, reboot more often, and clean memory-resident programs out of your registry that you no longer require. An old computer can have a ton of things loaded into it's memory on startup that are no longer required.
Virus software can tie up a lot of system resources and cause strange things to happen too...though I'd only recommend disabling that as a last resort, and definitely not permanently.
