Dependant on the modle of Serial to USB adaptor you have, the port number can change each time you use it. Some it is the same number every time. Hit and miss???
It is very much a case of trial and error finding which serial port number it has taken.
You could look in Control Panel. System, Device manager and look at ports. See which one is your adaptor. You can also assign different port numbers if you need (I had an item which would only accept ports 1 to 4).
Are you using the lead and adaptor supplied with the PT350 i.e. the jack plug to usb cable with the adaptor in the middle? If you are using our adaptor we will be able to work out what is going wrong. Did you install the driver for the serial/USB adaptor when you got your new PC (it comes on one of those mini CDs)?