NT, OS/2
I once had the fun of installing OS/2 from about 25 3,5 inch floppies and it was a Royal Pain In the Backside.
NT 3.51 was a breeze compared to that - insert floppy and press return a couple of times - done. Also, I remember reading some very funny stuff about OS/2 internals. NT demonstrated that Microsoft had learned quite a few things about proper operating systems and it probably was one of the best pieces of technology they ever released.
The whole Windows/Office franchise is based on this robust operating system that can also be secure, if properly used. That most users are using it the retarded way while believing in that Protection Scam (Virus Scanners), does not change that.
Running Unix as root is as insecure as running NT (including it's latest incarnation VISTA and Windows 7) as Administrator. Blame MS for not setting up a normal user by default, but don't blame the architects of NT for a lack of security.
Regarding VISTA - based on number of installations it certainly is less of a fail than all Linux distributions taken together. If you have 2Gig of RAM, it runs very well and is as stable as XP. My only complaint is that they changed the location of certain things more or less at random. The truth is that nobody needs the "innovations" of VISTA and win7 except the Chief Financial Officer of Microsoft. I bet XP will be used by the year 2020 in the millions.
And yes, the file system permission system UI is screwed up since the first version of WNT. But on a PC, this does not matter so much most of the time. Also, there are command-line tools to change the ACLs.