If you are using Logger Pro or Graphical Analysis (3.4 and older) and you also have the Palm Desktop software installed on your computer, the Palm software may be preventing Logger Pro from using the serial port.
You can free the serial ports by disabling the Hotsync portion of the Palm Desktop software. An easy way to tell whether the Hot Sync Manager is running is to look at the bottom right corner of the Windows toolbar. The icon for the Hot Sync Manager will appear as a red and blue circle with two arrows pointing toward the center of the circle.
To temporarily turn off the Hot Sync Manager, single right click on the Hotsync toolbar icon and choose exit. The icon should go away and the serial port will now be free.
To permanently turn off the Hot Sync Manager, single right click on the icon, choose Setup and click on Manual. This sets the software so that you need to open the program each time you want to do a Hot Sync.