I am getting the unknow device in device manager after installing circuit python. How to install driver for it.

Did you try installing the drivers from https://learn.adafruit.com/welcome-to-circuitpython/windows-7-and-8-1-drivers ?

Tried other way around for installing driver, this is the error I am getting.

As expected as the documentation clearly tells you that the signing certificate has expired:

Ok,thanks.

How to install this unsigned driver. As I have never install unsigned driver before. Can you help me out in it. 

It's just a warning -  so unless you have additional system policies in place, it should let you continue to install it. The 'Next' button isn't greyed out so it is allowing you to continue.

beacon_dave said:

It's just a warning -  so unless you have additional system policies in place, it should let you continue to install it

"Driver signature enforcement" is enabled by default in Windows and Windows will not let you use installed unsigned drivers. You either have to disable it on boot whenever you want to use the driver or disable it via group policy. There's also the option to use bcedit to disable integrity checks.

Wow... was that really as early as Win 7/8 ?

The last time I encountered it, it popped up the old security dialogue with  "Don't install..." / "Install anyway..." options.

beacon_dave said:

The last time I encountered it, it popped up the old security dialogue with  "Don't install..." / "Install anyway..." options.

I remember that being the case in Windows XP / Windows 7 and then they rolled out updates which locked it down.

I believe part of the reason was due to running promiscuous mode network drivers for WiFi that were modified among other purposes.

beacon_dave said:

The last time I encountered it, it popped up the old security dialogue with  "Don't install..." / "Install anyway..." options.

I remember that being the case in Windows XP / Windows 7 and then they rolled out updates which locked it down.

I believe part of the reason was due to running promiscuous mode network drivers for WiFi that were modified among other purposes.

One caveat for manojroy123 is that all 8.1 versions are now out of support and updates.
It may still be OK if you keep an existing version running, without installing additional software on it (and not exposing it to the internet too much).

But once you try (and fail) to install something additional, it 'll be hard to get support from either Microsoft or Adafruit.

I'm also wondering if Manoj did click that NEXT button....