Are you seeing COM ports numbered into double digits (COM78) because you plug USB-serial devices into different USB ports? If you are starting to have too many virtual COM ports on your Windows PC, this is how you can remove them.
Open a command prompt: Click Start > All Programs > Accessories and run Command Prompt.
In the Command prompt type:
…and press enter. You may have to restart Windows after this. Try without, first.
Run Control Panel > Device Manager. Clic on View > Show Hidden Devices.
Look under the Ports (COM & LPT) entry for unneeded serial ports. Right click on any that you wish to remove and select Uninstall.