I recently had to re-format my hard disk and, as a consequence, had to re-install my CS5 upgrade. I tried installing the disk last month and had trouble because it was originally upgraded from CS2 which Adobe no longer supports. Today I went in and re-read the instructions and downloaded CS2 from Adobe, re-entered the serial number, and still was told that it is not a valid serial number. I need a phone number so I can talk to a representative at Adobe and find out how to override this problem.
For CS2 you need to use the s/n as supplied on the website, NOT the one purchased.