The installation of Virtualbox is fairly straightforward. Oracle has done their best to make the installation process as painless as possible.
First, point your browser to http://download.virtualbox.org/virtualbox/vboxdownload.html and grab the latest version. Navigate to where you downloaded the executable package, and run it.
Click Next, and read and accept the license agreement.
This will bring you to a dialog which will list custom install options. For our purposes, it is best to use the default options, so just click Next.
After clicking Next, you will be asked whether or not you would like to create a desktop shortcut and a quick launch icon. Select your preference, and click Next to move on. This will bring up a dialog warning you that installation will reset your network interface. It is just letting you know that during the installation process, you will lose internet connectivity for a few seconds. Click Yes to proceed.
After clicking Yes at this dialog, we are finally ready to begin the actual installation. Click Install, and get on with it already!
This will proceed with a very typical install process, with your standard progress bar.
During the middle of the installation, "Windows Security" dialogs will pop up asking you if you want to install various drivers. These are necessary for your guest virtual machines to have access to things like USB and network interfaces. Be sure to click Install for all of these.
After a few of these security dialogs, installation is finally complete, and we can now proceed to the more exciting process of guest operating system installation!