If another virtual COM port is active on a system such as a IrDA port then the USB to Serial COM port will not be available.
If indeed another virtual COM port is active then the USB to Serial COM port may still be shown in the device manager however applications will not be able to find it.
In such cases, the user needs to disable the other virtual COM port in the device manager. After that the system may need to be reset and the driver may need to be reinstalled. However most of the time simply disabling the other Virtual COM Port will work.