This very nearly happened to me during the start of Cataclysm. My WoW installation became corrupt during the upgrade.
As a solution, I managed to copy the base WoW install from my son's computer, and only had to re-download some of the patch. It took about 4 days to recover (during glyphmass - and some may know how much I like my glyphs), and once I got into WoW there were already too many things for me to do.
I can help you with Windows XP (yes, an old operating system; but better than Vista that was offered when I last bought PC's).
If you are on Windows XP; copy your c:\program files\World of Warcraft folder.
- Take a copy now.
- Take another copy everytime you download a big patch
- If you have access to another computer, put your copy on it and run WoW from there.
As an aside; the patches are huge and consume both quota and network capacity; if you have 2-3 players in the household; you only need to download the patches once and can copy the patches to other computers; it is much faster.
Ideally; you will have:
- a current WoW installation you are using;
- a current backup on another hard drive
- a slightly older backup (say for the previous patch) not currently connected to your pc.
- have used a backup to restore WoW and continue playing it.