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Installing Vagrant
Vagrant, also by Hashicorp, is open source and used in simplifying workflows and configurations in virtual environments. Visit https://www.vagrantup.com/downloads.html and download the Windows version.
Once the relevant downloader is installed (in this case, Windows), install Vagrant.
Assuming that you already have VirtualBox installed, download the Metasploitable 3 source files from the GitHub repository at https://github.com/rapid7/metasploitable3:
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Once the source files have been downloaded, extract the files to a location of your choice. Launch PowerShell in Windows 10, change directories until you arrive at the folder with the downloaded Metasploitable files, and enter the ./build_win2008 command.
This should be enough to get you started with building your Metasploit 3 server. It is a very complex build for beginners but definitely worth a try.