I wonder if it's the "Bing Bar updater" service that does that. You'll probably want to look for that in "Computer Management" -> "Services and Applications" -> "Services". If there's any service with a name or description that starts with "Bing", you'll want to double-click it, click the "Stop" button, and then set the startup mode to "Disabled".

There may or may not also be Bing-related startup entries in your Windows registry. A tool like "Hijackthis" will zap those easily enough, or you can dig through the registry manually with Regedit if you know where to look. (That being "Software\Microsoft\Windows\Currentversion\Run" within both "HKEY_LOCAL_MACHINE" and "HKEY_CURRENT_USER".)