Go to Adobe.com. Sign in with your userid and password. Then use this link https://www.adobe.com/account/my-products-services.html
a list of your Adobe products should be retrieved. When it is displayed click on your CS 5 to retrieve your serial number. If that is the number that does not work for you. It may be because you have two current Activation on record. If this is the cause and your reinstalling because your machine had a problem that is now fixed but you were not able to deactivate Photoshop to free up an activation. You need to call Adobe and have them help you with the Activation process. This is a user forum user here do not have access to Adobe Activation servers and databases.