You have two free-of-cost options: wine ("WINdows Emulator"), and virtualbox. I used to run WoW in wine so I know it works. If you choose to run virtualbox, you will need installation media for whatever version of Windows you want to pretend to be running (because you will actually *be* running it), so I would try wine first if I were you.
I'm command-line oriented, so here's the procedure as I would do it; first, open a terminal, then enter each of these lines and follow the prompts:
sudo apt-get update
sudo apt-get install wine
At this point you will have wine installed. You should be able to simply install WoW at this point (or, if you already have it installed on a Windows box, copy the World of Warcraft folder over to ~/.wine ) and then run Launcher.exe from the WoW folder.
If you find that doesn't do it, contact me through the wowphiles.com
Web site and I will review my installation process and get more detailed notes for you.