WoW Cataclysm help!!?

Hey guys, I am back into WoW and I want the X-pac but heres the problem. My dad plays to and we have 3 computers. Before he got his Alienware he Downloaded cata to one of the computers and then DL it to his Alienware but I cant tell the other computer he DL it to. I checked this computers folders and it Said Expansion 1, Expansion 2, and Expansion 3 all .MPQs. I tried to DL cata to this computer but It wouldnt work because I found out that disc was the one my dad used. I dont know if that would make my computer Think I have cata but I really dont. I cannot access the other computer atm because there is no Router to it currently. I do not want to buy a gamecard yet until I figure out if this is the computer. The cata DL did not work though because it was a used disc so do not think I DLed it already I just want to make sure if it says that it truly is DLed thank you and Please be specific if its a long answer.!