A screenshot will help for your specific problem so that I can try to see if I can the same thing as you.
All driver files are located here: C:\WINDOWS\Temp\PL-2303_loggedDrv
But first you need to check what Device Manager says about the name of the device and if there are any yellow exclamation mark under "Ports (COM & LPT)".
If there is an issue you should be getting it is:
"Cannot Install this Hardware: There was a problem during the installation of this hardware: "USB-Serial Controller". An error occurred during the installation of this device. The system cannot find the file specified.
Before I say it is defective, I have to know if the device can successfully install drivers in Device Manager. A Multimeter can test for continuity going from all 4 pins of the USB connectors and to the needle, by using the resistance/continuity tester (the horn-like symbol). Connect up your black/red cables to the COM port/VΩmA port. If it only beeps/buzz, then there is continuity. If it shows up any constantly changing value and doesn't beep, then there is still continuity. If the value is near to 0/1, then there is no continuity and is damaged (unless it beeps). Meke sure you are touching both A-B points properly using the red/black cables.