World of Warcraft help?

I started playing Wow the other day with my older brother so i went out and bought the 1st expansion pack (The Burning Crusade) because i wanted to see if i like it because it was cheap. I have the original game and i used the activation key for the disc already so i went to install the game and is says "The file "C:\ Program Files (x86)\World of Warcraft\Data\common.MPQ" Could not be opened. If this problem persists, please contact blizzard Technical Support. (InstallerFile;;Open) and i looked on the tech support and im not sure what to do and i enjoy playing so i really want this to work any help please let me know ASAP thx:)