Changing the COM Port
It is possible to change the communications port number for a device in Window 7. This may be required if two serial (RS-232) devices are using the same port number. It is also useful if a device that is using a Serial to USB converter requires its COM number be in the range of COM1 or COM2 and there is no on-board serial interfaces in that range.
The following instruction are for Windows 7 however the instructions are similar for setting the COM port in Windows XP.
NOTE: Incorrect settings to the COM point may result in the disabling the input from a device. Before making any changes the current settings should be documented in case the settings need to be reset.
Start the Device Manger
Start Button -> Control Panel -> Hardware and Sound -> Device Manager (under Devices and Printers)
The Device Manager dialog box
Expand the Ports (COM & LPT) item and select the Communications Port (COMn) and select the Port Settings tab
The Communications Port (COMn) Properties
Press the Advanced button which will display the Advanced Settings dialog box.
The Communications Port Advanced Settings dialog box
The COM Port Number combo box will display the available COM port number and the COM port numbers that are currently in use. This will allow the selection of another COM port number.