Problem with uploading sketch onto the Arduino

After verifying my sketch, i proceed to upload my sketch onto the Arduino board.

However, i received this error message at times.

Serial COM Port 4 is already in use. Try quitting other programs that are running.

I closed every other programs already.

What can be the possible causes of this problem? :)

Do you have any bluetooth devices or software/services running?

No. Can the problem be the jumper is placed to Xbee instead of USB? :)

Can the problem be the jumper is placed to Xbee instead of USB?

There IS that possibility. PEBKAC problems are the hardest to debug on a forum like this.

Sorry to ask a random question.

The Atmega 168 chip of the Arduino Duemilanove is the entire microcontroller? :)

It's the brain. The rest of the board provides power to the brain, and connects the headers to pins on the atmega chip.