Since you have an Agilent GPIB controller, you may have to configure the appropriate VISA to not use Serial ports. I am not sure how you do this for Agilent VISA. For NI-VISA, you should go into Measurement & Automation Explorer (MAX) and either disable the Serial Passport (NiViAsrl.dll) or right-click on the offending port and disable it.
Dan Mondrik
Senior Software Engineer, NI-VISA
National Instruments
