My photoshop cs4 (which I purchased new online and have used for a year and a half), recently stopped working w/ error mssg that serial # is not valid. This followed taking my system in for virus removal. Does anyone have a suggestion for getting it running again. Using the serial # that came with the software and which has worked up til now. Adobe suggested buying new software. . . right now am having to use it as a trial version until I can get it running again.