virtualbox.org

All the settings are disabled. Please check screen shot. How can I enable for editting.

Make sure the guest isn't running.

The guest has to be fully shut down. Not running, and not suspended.

Most PCs react badly to having their motherboards modified while they are running...

Issue is I am not able to shutdown the guest completely after installing guest additions. I have read in a post to disable some settings. Which I am not able to do.

What happens when you go to "Machine/Close/Power off the machine" in the guest window ?
This should shut down the guest completely.

I am able to shut the machine as you said. But I doubt that it's not full shutdown/restart. Is it like Hibernate?

Its working. i am able to edit settings. Thanks.