Yes NT3.5/NT4/Win2k/WinXP were all technically multi user. However, to actually use them as a typical non corporate user, you pretty were pretty much guaranteed to be logged in as a user with admin permissions.

This was because for a long time Microsoft made little effort to make things easy for an end user to get things done logged in as a non admin user. They tried to improve this in Vista and made a mess of it, thankfully in Win7 that got a lot better*.

Things were even worse for developers, I still have to run Visual Studio as admin to do a lot of things (though they are fast reducing that list of things now).

* though it still doesn't really solve the problem for non technical users. There is a horrible irony where browsers and other Internet facing apps need to update themselves to keep the user safe, but to do so they need to ask the user for admin permissions if the user isn't logged in as an admin. To the uneducated user of course one dialog looks much like any other, meaning they can never tell a real update from something malicious also asking for admin rights. All of which leads to my mother emailing me screenshots of update dialogs on a weekly basis, asking if it is safe to press OK