The icon will not be displayed if you have previously chosen "Exit Ad-Free" from the start tray icon. In any case you can run Ad-Free Settings from your Windows Start Menu. Choosing "Ad-Free Settings" will re-start the Ad-Free software which in turn will re-display the icon in the system tray.

Also, in Windows XP the icon may only appear to be missing. XP has a "Hide inactive icons" feature. You can see this option by right-clicking on the task bar and selecting "Properties". The feature is normally activated and will cause inactive icons to disappear from the system tray unless the left arrow ( < ) on the task bar is clicked.