I started out as a Windows admin, with all Microsoft training and experience, and gradually migrated to Linux. Now I am 100% Linux, even my wife and family use Linux, and I work with Linux/UNIX for a living. Your mileage may vary, but the underlying concepts and technology were sufficiently impressive for me to completely forsake all of my training and experience and strike out in the opposite direction.
Regarding desktop environments: Linux supports any number of graphical environments, which can all happily co-exist on the same system. However, the two which enjoy the most popularity are Gnome and KDE. KDE stresses several values, but most importantly power, flexibility and constant improvement. In other words, KDE tries to make every release faster and use less resources than previous versions, while constantly adding new features. So, almost the exact opposite of Microsoft's design philosophy. ;) Gnome enjoys popularity primarily because of outmoded political factors. Specifically, KDE used to be licensed under a more restrictive license, so it was less desirable for free software purists (and maybe rightly so, at the time). However, since it's GPL'd now, that concern is obsolete. Gnome is slower and less customizable than KDE, so while it may be better and more flexible than Windows, I do not recommend Gnome. There are just too many good desktop environments for Linux that are superior IMHO. For example, XFCE is a gnome-like environment which is faster, lighter, and more flexible than Gnome. There are also a myriad of very light desktop environments that provide a lot with a *very small* footprint. Think "Usable desktop environment on my 486 computer" and you'll have an idea of what I mean. The "WindowMaker" desktop environment is fast, modern, and runs in 2MB of RAM. Impressive.
Regarding Linux Distributions (aka 'Distros'): During my 5 year affair with Linux, I've used every major distro and a myriad of minor ones. As a full time Linux user with a lot of experience, I really prefer Debian since it can be whatever you want in terms of an OS, it's just that flexible. However, with the freedom it brings comes almost zero hand-holding, so it really is a "RTFM" type of distro. If you're not into learning and studying as you go, Debian will chafe you badly until you're competent with Linux in general. If that doesn't sound like your thing, look into distros like Sidux, Mepis and K/X/Ed/Ubuntu, which are basically preconfigured Debian, albeit a bit broken in some fundamental ways. However, *ubuntu is eminently popular and even your grandma can install it. If nothing else, it may be a good leaping-off point for you. There are also lots of other easy-to-use distros which are not related to Debian, such as CentOS, Suse, Fedora Core, PCLinuxOS, Sabayon, and Mandriva to name a few. Experiment, try a few. Linux is free, and you're the one who has to use your operating system, not us. See what distro's the best fit for you now, and then learn the underlying concepts. As you learn and grow, your tastes will change. At that time, you can always choose a different distro!
Best regards,
-AT