Why Can’t I Download Burning Crusade For Wow?

okay heres the deal:
i decided to try out wow so i i bought the digital download for the price then paid the $15 monthly fee. then my friends told me to upgrade to the burning crusade. it had succesfully installed it, so when i click “play burning crusade” what it does is it starts “downloading” but then stops at 10% and does nothing. i have a windows vista and i know it has plenty space. i read online it might have to do with my comodo antivirus( i have COMODO internet security) or may be a problem with my router. if anybody has a solution it would be awsome cause all i want to do is play(havent played yet). thanks