"Serial Port COM6 is allready in use" message when downloading sketch

I get the error message "Serial Port COM6 is allready in use" when I try to download a new sketch. The previous project I run was the Arduino LOGO project, no. 14 from the Arduino Project Book that follows with the Arduino UNO kit. The project involves a sketch running in the Processing program with serial connection between the 2 sketches. I can see in the serial monitor that the Arduino board sends the bit stream on COM6 as required in Project 14. The Processing program is not running. I suspect that the problem arises because the COM6 port is busy when the Arduino IDE is trying to download the sketch. How can I stop/erase the sketch? If this is not the cause it must be a problem in the PC. Can anyone advice me on how to release/stop a communication port? I am running Windows 7.