Serial port already in use_error message

I sometimes receive an error message when I try to upload a program:

Serial port ‘Com3’ already in use. Try quitting any programs that may be using it.

The only thing plugged into my USB port 'Com3' is my Arduino board.

This message appears Intermittently. Sometimes shutting down and restarting the IDE will fix it.
Occasionally the sketch will not upload and the IDE will give the message" avrdude: stk5l900_getsync(): not in sync: resp=0x00"
Not sure if this is related or is a different issue.

Is there a way to avoid this problem?
operating system: Windows 8.1
board: Arduino Uno Rev 3.
Arduino IDE version 1.0.5

You might look through the background processes you have running to see if there another application that might use the serial port.

For example, some previous-generation cell-data cards would appear as virtual serial ports. Their software would scan all available ports and assume anything it found was their hardware.

OK, thanks, James.
I will look for other processes that may be using the port.
I noticed that Zohar has posted that he is getting the same AVRdude error message:
avrdude: stk500_getsync(): not in sync: resp=0x00


When I worked with my USB problem I catch this message too and this message "avrdude: stk500_getsync(): not in sync: resp=0x00". Somewhere was information that uninstalling IDE and installing it again might fix your problem. Like it sometimes help with driver problems.

I believe that some PCs uses funny USB port standard, try other ports if reinstallation don't solve problem. My all problems fixed when I use motherboard connected USB port.

avrdude: stk500_getsync(): not in sync: resp=0x00

Is a generic error which is probably caused by 100 things (okay maybe a dozen, but whatever). Nosingle definitive response can be given to solve it.