ITCanopy

Guest Additions includes device drivers and system applications that optimize performance of the machine. Start the guest OS in VirtualBox and click on Devices and then Install Guest Additions. It will mount the drive. Double click and start the setup. Click Yes and setup will be launched. Click Next and follow the installation procedure. After everything has completed a reboot is required.

I am using Windows 7 as guest OS in VirtualBox. Follow below steps: Start your virtual OS in VirtualBox. From the menu select Device and select Install the Guest Additions. Install Guest Additions Continue until the installation finishes. Restart VirtualBox and start your virtual OS in VirtualBox. From menu select the full screen or use short key in my case it was "right Ctrl". Below is optional: Go to VirtualBox click on File and open and Preferences... and click on Display and the settings to below: