i believe fizzler is right. 15GB is standard for a wow installation after patching to the latest version. if you have a nice spare hard drive, you could always use that, or partition it and use it solely for Wow.

Or you can always delete non-wow files you do not need anymore.

and check out ccleaner application, great for cleaning out old files you do not need anymore. also check to see if you have a lot of old system restore files you do not need anymore(if it is turned on)

otherwise invest in a 1TB hard drive, $99 USD and partition it to suit your needs