CDC-232 creates a virtual COM port on PC that doesn't have real RS- 232C port. It enables RS-232C communication (without control lines), after connecting the device and installing the driver.
Write the program to AVR, build the circuit, and connect the device to PC's USB port. Install the driver on Windows. Access the device through generated virtual COM port from terminal software or your application. Control lines (DTR, DTS, RTS, CTS) are not used by the host application. Set the terminal software as "no flow-control".
Windows requests the driver installation again when connected to other USB port. Detect the previously installed driver automatically. Another COM number will be assigned. If you set serial number in AVR (rebuild with modified usbconfig.h), you can get the same COM port at any USB port. However, you cannot connect multiple CDC devices of the same serial number.
Before detaching the device, close the COM port in terminal software or in your application. Otherwise, you cannot connect to the device again because of the broken file handle. Restart the terminal software or your application then. Switch to the fast transfer mode using "lowcdc.vbs" to get the baudrate higher than 9600bps.
Schematics and code here.