Some Javascript based PC and old computer emulators:

PCE http://www.hampa.ch/pce/about.html
PCEjs https://jamesfriend.com.au/pce-js/ The above ported to Javascript by using Emscripten

• Atari ST
• IBM PC 5150/5160
• Mac Plus - Emulates Macintosh 128K, 512K, 512Ke, Mac Plus, Mac SE or Mac Classic
• rc759 Regnecentralen RC759 Piccoline, which I guess was a computer made in/for Denmark specifically

PCjs http://www.pcjs.org/
This one is specifically an IBM PC 5150 emulator. Has several disk images, mainly DOS but there's a Windows 1.0 too.

jslinux http://bellard.org/jslinux/
This one is an x86 emulator written in Javascript that loads up a very simple Linux system when you visit the page. It seems pretty responsive. The guy that wrote it said it should be about the level of a 486 but without a floating point unit. No network support.

v86 http://copy.sh/v86/
This is another x86 emulator written in Javascript. This one has several disk images of Linux, DOS and even Windows 1.01.

Em-DOSBox https://github.com/dreamlayers/em-dosbox
DOSBox is a x86 emulator that comes with it's own DOS like environment. The development has been mainly focused on getting games running, but it can also run other applications. It's possible to run Windows 3.1 and even Windows 95 on it. However, Windows 95 isn't super stable. Em-DOSBox is a port of DOSBox to Javascript using Emscripten. Emscripten can compile C/C++ code to Javascript.

Internet Archive DOS and Windows games
DOS https://archive.org/details/softwarelibrary_msdos_games
Windows https://archive.org/details/softwarelibrary_win3
Windows 3.11 in your browser https://archive.org/details/win3_stock
Here's the thing that blows my mind. I ran Windows 3.1 on a 486sx 20Mhz in the early 90s, so about 23-24 years ago. Now it's in my browser and it's not even slow.
This Em-DOSBox generally has a better interface than the other x86 emulators above.

Windows 95 in your browser https://win95.ajf.me/
I'm not sure about the legality of this one. From all accounts it is buggy and tends to hang. I'm sure the Internet Archive version of Windows 3.11 above is legal. For one thing, they don't let you download it. All you can do is stream it from their website.

It's conceivable that someone could make an adventure game set in the 90s with fully working emulated computers from the era, given that all of this stuff works in the average browser these days. Even better if they can simulate a modem and let you dial up to a virtual BBS. It'd be great if Microsoft came out and said that MSDOS and old versions of Windows, such as 3.1 and 95, would be freely available for this sort of thing. After all, "It's a new Microsoft."