P L E A S E N O T E !!!!!!!
Are you using Windows 7 64 (probably will have the same problem in any Windows 64 OS)?
If so, you must uninstall CS2 completely (not just PhotoShop) and specify that the preferences are to be removed as well. When you re-install, the installer will give the default install location as something like "C:/Program File (x86)/Adobe" but, when you click "OK" install will respond that the location is not valid. DO NOT RESPECIFY AS "C:/Program File/Adobe", that's the location for 64-bit apps and CS2 is 32-bit and needs to be in the (x86) folder to work properly. Replace "C:/Program File (x86)/Adobe" with "C:/Program~2/Adobe". The installer may not "light" the OK button. If so, click the browse button then the cancel button and "OK" should activate. That's it, CS2 will install. If, after the install completes, you get the message that the app may not have installed properly, you probably did something wrong (i.e., when it's in the 32-bit program folder you won't get this message).
Remember, this fix ONLY APPLIES TO Windows 64-BIT OSs. There are other causes for the message "Your adobe photoshop user name, organization, or serial number is missing or invalid. The application cannot continue and must now exit.".
I spent an hour and fifteen minutes waiting on hold for a Adobe technician for help only to be advised that CS2 is no longer supported by Adobe. Don't waste your time trying to get help from Adobe or getting upset with them for not being helpful (what's the point, they're holding all the cards and want you to drop another bundle for their latest release).
