It's simple, DO NOT TOUCH 4.3.2!
WoW have changed the DBC layouts in the 4.3.2 patch, making them incompatible with all exe versions except 4.3.2. This breaks compatibility with all Cataclysm private servers and connection will be impossible if you upgrade.
For those that upgraded already and want to get on, you need to download the 4.0.6 game files. Here is a link to a torrent. (You need a program like BitTorrent to open it and download the game):
Just download that into a folder, download the IceCold Launcher and put it in the folder, run it as admin and login :)