When you run the virtual machine on VirtualBox and you click on virtual OS, the virtual machine automatically captures your keyboard and mouse as if it is being used only in that OS. This makes the keyboard and mouse unavailable to any other application on the host machine. If this is not what you wanted, follow the tips below:
To get the control back to the home machine, you can press the host key(Normally this is the CTRL key on the right side of the keyboard). This will return the keyboard and mouse(whatever is already captured) to normal operation. If you want to share your mouse between the virtual OS and your home OS without pressing the right control key, you must install the Guest Additions from the virtualbox menu.
After starting the virtual OS, press the right control key to get the mouse out of the virtual OS control and click on the Devices menu. Select Install Guest Additions. This will install Guest Additions software in the Windows virtual box. Follow the prompts and once installed, reboot the virtual box and login. You'll get a warning message about the mouse control. Click the OK button and continue.
Once you have installed the guest addition, you can now move the mouse in and out of the box without pressing the host key.