I've purchased a new PC and am looking to deactivate Adobe Design Premium CS5 from my old PC, so that I can activate it on my new PC.
However!!... I don't know the original serial number?
I understand that I require a 24 digit serial. The serials under the HELP menu of Illustrator and Photoshop only give me a 16 and 20 character code.
If you registered the software when you first installed it then you might be able to find the serial number thru your Adobe account online.
If that doesn't help and your PC's is a Windows machine then try running Belarc Advisor
You are allowed to have two activated installations if you would like to retain the original installation as a backup.