Shoutbox

Completely uninstall Messenger. Make sure there are no remnant files or folders.

Then go to Start, Run and type in %temp% and then OK. (Not sure if that shortcut works on 2000, but it should. If not, manually go to your temp folder)

Delete everything in the folder, then reinstall Messenger.

If for some reason it won't let you delete some files, do so in Safe Mode, or failing that, use something like MoveOnBoot or Pocket KillBox.