Though Linux is highly based off Unix, it's still a different critter. I can take FreeBSD (which is a Unix variant, based off the Berkely distro) and pop X11 on it, viola, I have a GUI
As for Windows <-> Dos...same thing. the WinNT kernel is not the same critter that DOS was.
To answer your question...Get some live CD's, they will allow you to try out Linux without even installing anything. Full usability, without the installation. Knoppix is a very powerful Live CD, as well the others listed above Ubuntu, SuSE, Mandriva, etc all have LiveCD's, free to download. You will need an ISO burner, I suggest Alcohol 120, but ImgBurn does a fine job as well.
Me, I prefer Mandriva 2007 with KDE, preference is much the case though. SuSE is a fine OS as well.