CS2 activation servers' shutdown: In January 2013, Adobe Photoshop CS2 (9.0), with some other CS2 products, was released with an official serial number, due to the technical glitch in Adobe's CS2 activation servers (see Creative Suite 1 and 2).
The solution to this problem is the Account Management page of Creative Cloud. You can carry on the deactivation process from there, or get in touch with Adobe Customer Service to use your serial number and do it on their end.
The serial number is assigned to you when you purchase Photoshop from Adobe. To view your serial number, log in to your online Adobe account. All your serial numbers will be viewable there. However, you may not see these numbers. If that happens, try registering your product and reload the page.
There is one more way to find the serial number. You can use Belarc Advisor (Windows) to retrieve your serial number. Or, if you are using a Mac OS, try using the Product Key Finder on Mac. These utilities will help recover activation keys for all the programs on a system.