Comment by jasomill

Even if VMs do not, whole-system emulators surely will.

But Windows 95 would be a poor choice, as you'd be stuck emulating a bunch of very specific aspects of both Windows (non-Unicode APIs, 16-bit application support, etc.) and the PC archictecture (BIOS, A20 gate, etc.) that exist only to maintain backwards compatibility.

If Windows is a requirement, you'd be better off building against something like the subset of Win32 APIs that has been stable since Windows Server 2003 and ideally testing against Wine, contributing patches as necessary.

If it were me, though, and it was something that required a general-purpose, non-realtime OS, and wasn't so performance-sensitive as to require deep integration with platform-specific APIs, my first thought is to target a Linux-compatible subset of FreeBSD, which should be enough to get you most of POSIX and then some, plus a full-featured GUI if necessary, portable between two open-source OSes that run on a wide variety of hardware.