Microsoft kindly provides a number of Windows VM images free-of-charge to help with testing websites and applications on Windows. Many of these VMs have limited-time licenses and they'll expire if you don't add a valid license key (maybe pull a valid license key off an old computer you're throwing away?).

• Windows 7, 8.1, 10 Virtual Machines - for testing websites, but supports installing other applications
• Windows 10 Enterprise Developer VM - has Visual Studio, Windows Linux Subsystem
• Windows 10 ISOs - CD images from which to install a fresh version of Windows in a VM
• Windows XP VM - Instructions to download a legit copy from Microsoft (the VM image is designed to be used with Windows 7's XP Mode feature, but was useful to me when a client had an appliance with a web app that only supported IE 6)