Old icons will live quite happily in the tray area forever without affecting anything.
CCleaner gives you an option to clear the System tray:
You can also do it manually with a registry hack by saving this code as a reg file and merging it into your registry.
With the manual method you have to stop and re-start explorer.exe which can be done through the Task Manager (R-click taskbar for an open option) or re-starting the PC. Be aware that both methods restore default settings to all the notification area icons so your display preferences will need to be re-set (Remember to un-tick the CCleaner method after it has run!).
Windows Registry Editor Version 5.00
Footnote In my experience, on Windows 7 the CCleaner method of clearing the notification area is a bit hit or miss. The manual method however, works every time.