By far, my favorite distribution is Debian, and my favorite window manager is WindowMaker. For people that are used to Windows, however, and are not necessarily looking to make the switch, you would be well-served by using a distribution and GUI environment that they'll find intuitively familiar.
That being the case, I would recommend KDE or IceWM for your GUI environment, and an easy-to-use distribution that uses KDE and/or IceWM as its default.
Distributions that fall into this category numerous, but those that have the best hardware support and most polished package systems are SuSE and MEPIS. MEPIS is a LiveCD distribution that is extremely easy to install on the hard drive as well, though as a LiveCD distro it is not quite as well suited to permanent installs as those distributions that were designed primarily with that in mind. As such, all things considered, I recommend SuSE for your purposes.
Fedora is a good distro, if you're into kitchen sink distros. For your purposes, it sounds like a kitchen sink distro (such as SuSE, MEPIS, Fedora, et cetera) is exactly what you need. Unfortunately, Fedora uses Gnome as its default, which will not be nearly as intuitively familiar to Windows users as KDE. If you were dealing primarily with Mac users, on the other hand, Gnome would be the way to go. Ubuntu is, by all accounts, an amazing distro as well, but also suffers the problem in this case of using Gnome as its default GUI environment.
Mandrake is one you'll probably get a lot of recommendations for, but I will advise against it. For one thing, its design is based on some very odd "user friendliness" philosophy, and it is sliding ever further toward some kind of hostageware approach to marketing.
There's also Knoppix, wich is a fantastic tool for converting people, but probably doesn't suit your purposes as well as MEPIS. I guess I'd make Knoppix the third-place recommendation after SuSE and MEPIS.
All of that having been said, if you have the time and inclination, using Debian to set up exactly the operating environment you want might be a best option. If your end users are never going to have to deal with initially installing the system, it's a great idea to use Debian, assuming you have the familiarity with it to be able to set it up effectively without any major difficulties. Debian is much easier to install these days than it was in the past, and it has always been one of the easiest to manage and tweak to suit your preferences (because of the apt tool). This distro simply requires you to know what you want, and to be willing to put a little time into setting it up. It requires slightly longer to get a system just right in Debian than in SuSE, of course, but on the upside SuSE is always "just right" according to what Novell thinks you should use, rather than according to your own tastes.
Every kitchen sink distro (meaning: it includes everything, including the kitchen sink) does things the way others think it should be done. Lean distros like Debian, Slackware, and Gentoo are designed to do things the way you want them done, with little or nothing in the way of system defaults that will screw with your preferences. The trade-off is that it takes longer to get the "perfect system" than it does to install a fully-featured system that is "good enough".
