Oh I have been known to go on a tangent or two myself...
But yeah! I too had DOS on my first computer as a teenager.
If you want a similar writing experience, you might try a more modern program like Write Room that blacks out your screeen and you can write like the ole dos days.
I'm still choosing between plain old Word (well, OpenOffice in my case) or Scrivener. But knowing me, it'll probably be Scrivener. It has a build-in fullscreen modus without distractions.
If you want just a blank page and a cursor (without being able to change anything, like font or writing in italics or such) you could try DarkRoom (or WriteRoom if you're on a mac).
WriteRoom and its Windows-equivalent Dark Room are the complete opposite of what everyone here has been suggesting. The program opens up in full screen with no menus and a black background, effectively shutting out distraction and clutter. I'll let you all decide if it's something that suits you or not.
Here are two computer apps I know of: WriteRoom: Distraction-free writing for Mac, and Time Doctor for work related things (a fellow ADHD coworker showed me that one.)
If you have a mac, I'd recommend Writeroom. It's very useful in full-screen mode if you get distracted easily.
For formatting, it's not so good, but for practically anything else, and especially doing free-writes, it's great.
Strange Little Hangups is a country, and I am its King!
At times, Fountain is the only thing that lets me make pages. Simplicity defeats anxiety. Just write something. Start and keep going.
Some cool apps: