Depending on your USB to serial converter, you would need to enter "Device Manager" and find the device it installs.
Right click - My computer > Properties > Device Manager
Try leaving device manager open while you plug in the adapter. You should see a new device pop up.
Open the properties for that new device, and somewhere in those options you should see a place to assign the adapter to an open COM port. Assign it to a port that you know isn't being used (like 8).
Then you can go back to your communications program and choose that port and it will use the USB adapter.
Let me know if you need something more specific. If so, I'll need some information from you like screenshots, etc.