Actually, I think you need to get stuck at a command line trying to figure out what to do next a few times.
I'd recommend CentOS - it's basically RedHat Enterprise Linux for free. When you get to a corporate network you'll almost definitely see RHEL. I'd also suggest you install the minimal installation (you only need the first CD for this) and build up your GUI from the CLI - this will invariably teach you something of how *nix works. If you really want to go nuts, download the source for everything you need, build RPMs from them and use those rather than "yum install package" - this is the "right" way to install packages on an Enterprise level RPM-based distribution. Even if you don't use CentOS, you can do this with Fedora as well - Fedora is the testbed for the next version so this is a viable option as well.
You don't need something user friendly at this point. You're not going to learn anything about the internal system if all you have to do is open firefox and thunderbird - which you could do in Windows anyway. If you really want to learn, you need to experience the command line. This is where all the power lies in Linux, not with the pretty graphics.
If you're going to get a BSD, just get FreeBSD - for much of the same reasons. It may not be as "easy", but it will teach you more about the underlying system. Some other Distributions that are good for this include Slackware, Gentoo and Debian.
If you do happen to install Windows after linux - destroying your Linux bootloader, all you need to do is boot up the Linux install CD and select the "Repair" option. This will take you to a screen where you can reinstall the bootloader. There's a much more involved method of doing this as well which can be performed from any Linux bootable disc (CD or floppy), but I'll leave that alone for the moment.