Booting the already installed Windows from the VirtualBox installed in Ubuntu

It can be done, but it is not recommended if you don't really know what you are doing:

Warning

Raw hard disk access is for expert users only. Incorrect use or use of an outdated configuration can lead to total loss of data on the physical disk. Most importantly, do not attempt to boot the partition with the currently running host operating system in a guest. This will lead to severe data corruption.

Anyway, you should look into VirtualBox documentation, in particular in Advanced storage configuration::Using a raw host hard disk from a guest.

You can not directly run the Windows installed physically on a harddisk partition or on a different disk.

However, you can migrate the Windows installed on your physical computer to a VirtualBox virtual machine. I can not easily summarize the procedure because it is a little bit complicated, so yo can read the official documentation here: https://www.virtualbox.org/wiki/Migrate_Windows

It can be done. I actually reinstall Windows on my hdd from VirtualBox while running Linux and then I am able to boot it normally.
Be careful though, mount your Windows partitions on Linux as READ ONLY so you don't mess things by trying to modify files from the 2 OS-es at the same time.

Here are two useful links:

https://forums.virtualbox.org/viewtopic.php?t=33356

http://www.rajatarya.com/website/taming-windows-virtualbox-vm