If your System event log has errors for an non-existent COM port, you may have an erroneous Serial# sub-key at:
Using Regedt32, double-click the Paramters key. Select each Serial# sub-key and check the DosDevice value in the right hand pane for a valid COM port. If you finds an invalid or duplicate COM port, select the Serial# sub-key and delete it.
NOTE: If you find a Serial1000# sub-key, chances are it was created when you Cancelled an Ports entry in Control Panel.