Investigate what programs are starting up when windows starts, i.e. things in the startup part of the start menu and in the system tray next to the clock. Try changing their settings so that they don't get started when windows does and see if that makes it go away.
Particularly, look for any references to programs that you have removed, but that had some kind of startup alone with windows as in system tray icon, etc.
My money would be on this being caused by you uninstalling something, and there is a bug in the uninstaller. Something is left over and trying to start but can't. But don't forget about the possible presence of viruses.