I published an article about Long Gone Icons in Windows before and walked you through a manual way of removing those icons from the system.
Every icon that appears in the system tray is cached by Windows and will remain in that cache even if you uninstall the application completely from your computer.
This may reveal information about those programs to users who have local access to your PC and clutters the Notification Area control panel applet that lets you select which icons to display in the system tray area.
While that is usually not a problem, it can be in certain situations. Icons that encryption software add to the notifications area may reveal that data may still be encrypted on the device.
You can take a look at all cached items on your system if you like to see that they can reveal information about your system even if you thought that you did leave no traces of an application behind.
Right-click the Windows taskbar and select Properties from the menu. Now click on the Customize button which opens the Custom Notifications window.
That window is divided into current items which are those that are currently active on your system and past icons which are the icons that are stored in the cache. This includes icons from applications that are still installed on the system but also from those that are no longer installed on the computer.
Past Icons Flusher is a software that clears those past icons automatically which means that you do not have to go into the Registry and remove them manually. The program asks you during execution if you want to flush the Past Icons, a click on Yes starts the process while a click on No cancels it.
Please note that the explorer instance is killed in the process but it should restart just fine after the Past Icons have been removed.
Update: Please note that Past Icons Flusher does not appear to be available anymore. I suggest you use Tray Cleaner instead.