on boot, mouse cursor jumps to taskbar and doesn't work

on windows 2000, with language support for Japanese, the
use is mostly the browser and outlook for mail. PC worked
fine for long time, now I have a problem with the mouse.

On boot, the mouse looks ok, I can use it for logon. But
partway through the boot process, the mouse cursor jumps
down to the taskbar and is stuck there.

I've tried different mice. I've uninstalled the driver and
reinstalled. I bought a new mouse and installed a new
driver. I've downloaded all the updates possible for the
OS. The new mouse worked for awhile, but then reverted to
the same problem. I've checked for viruses, but Norton is
updated and can't find anything. We haven't installed any
new software either.

Any ideas?