Downloading VirtualBox

Google "VirtualBox Downloads", and click the most reasonable link.  This will take you to a page similar to this.
According to Oracle, a host is "the operating system of the physical computer on which VirtualBox was installed. There are versions of VirtualBox for Windows, Mac OS X, Linux and Solaris hosts."

 Click the link that corresponds to your host operating system.  For Windows, the download will be a .exe file.