Photoshop CS4 stopped working
 

First off, this is a legitimate copy. It was given to me as a gift and was bought (from Amazon I think) righteously. I have all the packaging so its not me trying wonder whats wrong with a pirated copy.

For over three years PS CS4 was working perfectly. Then earlier this week, my computer completely crashed. I did a clean install of the OS (Windows 7 64 bit) and began reinstalling my programs.

Suddenly the serial number for PS CS4 is invalid. I call tech support and Gupta tells me there is nothing then can do about it unless I provide proof of purchasing. Well I don't have it. At least I had a "30 day trial" when I installed it. Now it claims my license is expired and immediately closes the program. I've done uninstall and reinstall a dozen times and still the same problem.

I actually would intend buying an upgrade but now not sure if that would even work. Is there a way to delete the registries that seem to have imbedded itself that I'm trying to install an expired copy?

Photoshop keeps track of installs. You will need to "unregister"/deactivate first and then re-try. They track installation and if he installed then you did you have likely exceeded the limit.