How to use Arduino to control virtual com port?

Hi,
I have a device that uses USB to connect with PC. This device is recognized COM (virtual com port) device by PC. It is easy to use C# to control this device, because the interface is COM port like RS232 (serial port).
Now, I'm going to use Arduino to control this device. I bough a USB host shield, but I don't have any idea to control this device.
If anyone has any idea, pls share to me.

Sincerely,
Darren

If anyone has any idea, pls share to me.

Without knowing what the device is? Not a hope in hell.