When the USB to TTL cable is installed windows assigns a COM port number for the device.  To find out which COM port number is assigned to the Device follow the steps below.

1. Ensure that the USB to TTL cable is connected to your PC
2. Open 'start | Control Panel'
3. Select 'Performance and Maintenance' if using category view (otherwise ignore this step)
4. Select 'System' to open the System Properties Dialog box
5. Select the 'Hardware' Tab
6. Select 'Device Manager'
7. Expand the 'Ports (COM & LTP)' Tree to list all the available Serial and Parallel ports on your PC.

There should be an entry 'USB2TTL Serial Port' with the COM number in brackets.