There are hundreds of MUDs but only a few dozen have concurrent players online in the hundreds.
Some of them have web clients you can play in your web browser too like the one I was playing called Genesis LPmud which you can play in your browser without installing anything. I recommend Genesis LPMud because it has an ascii art map, a tutorial island and because it is non-profit with no microtransactions and is not pay to play.
Discworld is pretty good too, based on Terry Pratchet’s novels. They have their own client too with maps and toolbars etc… But there are also clients like Mudlet and TinTin++(terminal based) which can be used to play any MUD but with these you have to install scripts to auto-map or draw your own maps on paper like when playing Dungeons and Dragons.
As for anime MUDs there are a few Dragonball Z ones and a Macross one in which you pilot a mech. There are also a few Final Fantasy themed ones.
Most MUDs are tolkien/dragonlance fantasy though there are some cyberpunk and a couple of space exploration MUDs too. All of them will run on win 7. I mean they are text based games created in the 80s, Genesis LPMud has been online and developed since 1989, so they will run on anything.
There are roleplay-enforced, roleplay-encouraged and non-roleplay totally hack and slash muds. MUDs are basically the closest you can get to playing DnD in an online game. The reason for this is because of how others will see the commands you type when you are in the same room.
For example if you type whisper X good morning then player X will see “X wisphers good morning” whereas player Y will only see “Z whisphers to X” there are also hundreds of emotes or player actions because they only need to be coded with text without the need for any assets. For example if I type bow graciously X then my character would bow graciously to X. Basically because both the input for commands and the output is in the same medium of text it is easier to get into it, a bit like a poor man’s virtual reality lol.