Many within the DFIR community make use of virtual systems for testing...for detonating malware, trying things within a "safe", isolated environment, etc. However, sometimes it can be tough to get hold of suitable images for creating that testing environment.
I've collected a bunch of links to VirtualBox VMs for Windows, but I cannot attest to all of them actually working. But, if you'd like to try any of them, here they are...
MS Edge developer virtual machines (Win7 - 10, limited time)
Windows 7 Image, reports no activation needed
Win95 virtual machine
Various MS virtual machines (MS-DOS, Windows, etc.)
Windows 11 Dev Environment (eval)
Use Disk2vhd to create a virtual machine from an existing installation
ReactOS - clone of Windows 5.2 (XP/2003)
Back in 1994 and '95, while I was in graduate school, I went to Frye's Electronics in Sunnyvale (across the street from a store called "Weird Stuff") and purchased a copy of OS/2 2.1. I did that because the box came with a $15 coupon for the impending OS/2 Warp 3.0. If you'd like to give the OS/2 Warp OS a shot, you can try this v4.52 download, or try this site for other versions of OS/2.
Hope that helps!