Pardon my lack of Italian past ordering another glass of wine ....
The error message you are getting can come from two places:
1) You still have the Arduino IDE connected while trying to run Pronterface
2) You still have Pronterface running while trying to flash the firmware with the Arduino IDE.
In both cases the solution is the same - only run one program at a time.
The problem can also come from another place:
1) You need to load a driver on the PC to talk to the Arduino
2) The driver connects to the Arduino via USB
3) The driver looks like a serial port on the computer
If either the driver is not loaded or you don't know the port, you will get that error. On a Windows machine, you can find the serial port ID under the com and lpt ports section of the devices tree.
