I added a 4 port pci serial card to my windows 7 PC. I want to assign them COM1-4.

I know how to change the COM Port Number when the com port number is not already in use.

In my case, COM4 is taken by something... probably some type of usb to serial device I connected at some point. If I knew what it was I could connect it and then change the com port through the Device Manager -> Ports -> dialogs.

But, I have no idea what the device has reserved com4. I'm assuming there is a registry setting or some device descriptor file somewhere that I need to remove. Can anyone point me in the right direction?

2 Answers 2


"Method Using Elevated Command Prompt"

From Start Menu type cmd into search bar right click on cmd.exe and select run as administrator

at UAC select yes to allow Windows Command Processor Program to make changes.

At the Command prompt type


Then to open device manager type

DEVMGMT.MSC (Followed by enter)

In device Manager select View / Show hidden devices

Expand Ports (COM & LPT)

Hidden ports reserved will be shown with greyed out symbol & associated manufacturer / device type

screen shot device manager ports

To re-hide non present devices in future - from an elevated Command prompt type


  • Looks super promising! Can't wait to try this. Thanks Antony.
    – blak3r
    May 9, 2012 at 17:26
  • You are the man. I've come back to this post like 4 times already. I wish I could upvote you more!
    – blak3r
    Aug 4, 2012 at 0:39
  • thanks, glad to assist, I remember when I first found out the solution.
    – Antony
    Aug 17, 2012 at 6:34

(For those who came from google) If the previous answer doesn't work and has installed Windows mobile 6 SDK. It may be problem with XPVCOM.SYS (this driver reserved 14 COM ports) To uninstall in the console:

CD \Program Files\Windows Mobile 6 SDK\Tools\Cellular Emulator\
InstallXPVCom.exe UnInstall 

Then reboot.


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