PySerial could not open port COM4: [Error 5] Access is denied - please help

[Adam]

Host OS:    Ubuntu 10.04 LTS
Guest OS:    Windows XP Pro SP3


I am able to open port COM4 with Terminal emulator.

So, what can cause PySerial to generate the following error ...

C:\Wattcher>python wattcher.py
Traceback (most recent call last):
  File "wattcher.py", line 56, in <module>
    ser.open()
  File "C:\Python25\Lib\site-packages\serial\serialwin32.py", line 56, in 
open
    raise SerialException("could not open port %s: %s" % (self.portstr, 
ctypes.WinError()))
serial.serialutil.SerialException: could not open port COM4: [Error 5] 
Access is denied.