look in the device manager with the radio and cable connected and see if you have any COM ports listed. One of those will be the port that the radio is connected to. You have to tell the software to use that port in one of the menus. If a com port is not listed in the device manager, you may need drivers or something for your programming cable. Those usb cables are typically a USB to Serial adapter.
I do recall with the prolific driver, you would need to locate the exact one that would work. Most the time the driver provided does not work.
I forget where I found it but a true prolific driver when I did a search I had to try a few before I had any luck. It depends on the version I would have to check what version I had used to get one of them cables to work.
It was more of a experiment. It did read and write. Figuring out the actual prolific driver was the fun part.
You would also need to be sure your cable has a true prolific chip.
You will need to find an older Prolific driver and install it, rather than a newer one. That is what worked for me.
If memory serves me correctly, it was version 3.0.0 or less. I will have to look when I get home.
I use a true serial cable for the HT 750 and 1250 with XP, as well as other Motorola radios. Software designed to work through a true com port does not seem to translate well into a USB port.
I like to call it the old nintendo trick. Sometimes the cartridge a little hot air, breathe on it a little bit like you would to fog up your glasses to clean them. Then try. Ensuring that the contacts on the accessory flex are clean and intact is key.