How to access windows file system in Ubuntu Virtual Box

You need to create a shared folder.

Open Ubuntu virtual machine settings. Select Shared Folders > Add new shared folder.
Add the path of your Windows folder you need to access on your Ubuntu VM and click OK.

Using Virtual Box Auto-mount option :

In your Ubuntu machine, install Virtual Box Guest Additions

When it is done, open a terminal (Ctrl+Alt+T) and add yourself to the vboxsf group (VirtualBox shared folder group) :

 sudo adduser $USER vboxsf

Reboot your Ubuntu virtual machine and you will find your shared folder under /media/sf_MySharedFolder.

Without Virtual Box Auto-mount option :

In your Ubuntu machine, open a terminal (Ctrl+Alt+T) and create the folder where your want to access your shared folder (I am going to create under my home directory:

mkdir shared

Then mount your shared folder :

sudo mount -t vboxsf $USER ~/shared/