ReInstalling 737-600/700

Several months ago I purchased a Win 7 computer and have successfully transferred all my FlightSim stuff to it. I have installed the standalone version of the PMDG 737-800/900 which works very well. I now wish to join a Virtual Airline but their rules state that I need to do my early flying hours in a 737-700. I had previously purchased (via download) the PMDG 737-600/700 but had deleted it from my original Win XP computer (but I had made a back up copy). When I try to reinstall this (on the XP machine as a test) I get a "corrupt air file" message plus I see 2 separate exe files, one for the -6oo and one for the - 700. Please can someone tell me how I should proceed.Many thanks,William Hobbs