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. 

8. Double Click on the USB2TTL Serial Port Entry
9. Select the Port Settings Tab
10. Click on Advanced
11.  The Com Port can be selected from the drop down list

Note that some Com Ports may be labelled as "(in use)" you can use these so long as the device assigned to that port is not connected at the same time.