First of all, is this a store bought copy? If it is, I would return it for an exchange. Also, if it's the 64 bit version and you are running the 32 bit version of windows, it may not work........or is it the other way around? With the cost of cs4, I would definitely return it for a refund or exchange. It is a great program, the best out there.......been using ps for years...good luck!