If you'd like to install additional virtual machines for general curiosity, all you need is the ISO for whatever operating system you want to run as a virtual machine. Microsoft Windows offers free trials of its server software at https://www.microsoft.com/en-us/cloud-platform/windows-server-trial. Alternatively, you can install additional versions of Ubuntu by downloading the appropriate ISO file from http://releases.ubuntu.com/. CentOS, which is essentially Red Hat Linux and is available at https://wiki.centos.org/Download. All of these operating systems can run as virtual machines in VirtualBox. I recommend experimenting with various Linux operating systems and see which one you gravitate toward the most. If you ever want to work through binary analysis against the Windows PE format, using the various available trial versions of Microsoft Windows is the way to go, especially on a budget for a home lab.