Add/Remove Programs simply runs a command. The command usually runs the item's uninstall program. If the uninstall program is no longer there or is corrupted, the item will not be removed or uninstalled.
The easiest thing to do if the uninstall program is no longer there or is corrupt and you still want to uninstall the item, is to reinstall it and then uninstall it.
If the item has already been uninstalled but it is still listed in Add/Remove Programs or you just want to remove the item from Add/Remove Programs, you can use the Add/Remove Editor in RegVac Registry Cleaner to remove it.