Not compatible

I haven't played in a couple of weeks but I wanted to log on and when I click on the folder I get this message:

' The version of this file is not compatible with the version of Windows you are running. Check your computers system information to see whether you need an *86 (32-bit} or *64 ( 64-bit) version of the program and then contact the software publisher'

As far as I know I haven't changed anything.

Any help would be greatly appreciated, thanks.

Did you, at some point, try to run the client with compatibility mode?
I would try redownloading Wow.exe from the torrent on the web, if it does not work the easiest thing you can do is to simply redownload the whole client.