I mean, there is actually leaked source code of Windows XP out there, because, you guessed it, they had a leak of that, too.

But I actually said "garbage code", because I didn't want to say that everything they've ever done is purely garbage. I didn't want to claim that I have particular insight into specifically their code.

I have to assume, though, that their code quality is garbage, because:

• Lots of MS software is buggy. In particular, all those security issues are bugs, too.

• They keep backwards-compatibility to just absurd degrees. To this day, you can't create a file that's called "aux", for example, because at some point, they had to block that to retrofit filesystem support into their OS.
  At the very least, this is going to mean they'll have tons of such workarounds and gotchas, which will make it difficult for new devs, but also offer more surface area for bugs/vulnerabilities.

• Well, and then there's some urban legends. For example, I've heard that the entirety of Windows is in one giant monorepo. I just quickly peaked into a supposed copy of the Windows XP leak and that did look the part...