The most common source of such errors is the port being already opened by some other application, or frequently by a previous (executed, but forgotten to kill) instance of your code. To test this hypothesis, the easiest way is to run some other program that opens the same port (for example Hyperterminal) when you're getting the error. If it fails also, try to find who's holding the port. Also, [Portmon][1] is a useful tool in debugging similar problems with the serial port. [1]: http://technet.microsoft.com/en-us/sysinternals/bb896644

The most common source of such errors is the port being already opened by some other application, or frequently by a previous (executed, but forgotten to kill) instance of *your* code.
To test this hypothesis, the easiest way is to run some other program that opens the same port (for example Hyperterminal) when you're getting the error. If it fails also, try to find who's holding the port.
Also, [Portmon][1] is a useful tool in debugging similar problems with the serial port.

[1]: http://technet.microsoft.com/en-us/sysinternals/bb896644