USB Cdc communication pic18f4550 with MCHPFSUSB

Hello everybody,
I'm trying to achieve this communication beetwen the pic18f4550 and my pc with a virtual COM port, and i'm using the firmware in the example of the MCHPFSUSB's microchip "tutorial".
Is working but i've always the same problem:
when i plug-in my board in a new pc it doesn't works...
I have to upload a different programm (like one that blink all the pin of the pic) and the re-upload the hex file with the cdc programm (from the MCHPFUSB example).
If I remove the board and re-plug into the same USB port, there is no way (for now) to make it work again... The only method i've found for now is to change the USB port and repeat the previous procedure, end at the end Windows recognized the RS-232 port simulated and requires the drivers.

This is frustrating and I don't know how to fix it, if someone can help me I'll be infinitely grateful...
I'm always here if you required more information,
thanks Matteo.