If you have used up your total installation allotments of Premiere and/or Photoshop Elements, you will need to contact Adobe Tech Support to have your serial number released.
Just user to user....based on past and current history, Adobe Tech Support is not going to help you with a technical issue which is preventing you from using your purchased serial number for Photoshop Elements. Version 12 is not the current version. And, the Adobe Tech Support is limited even for the current version which is 13. If this matter involves a purchased activated serial number trapped on a crashed computer while you are trying to install on a 3rd computer, then probably Adobe Chat Activation can be of assistance. If that is the case and you want to explore that possibility, then click on the following link
Adobe ID, Signing In
Adobe ID, Sign In, Account Help
If the above link does not hold its set, then you will need to navigate to the Chat Panel using the above titles as guides.
Are you getting one of those "too many computers with your serial number" messages? Then the above should apply to your situation.
Before the above consider....
a. For Photoshop Elements technical issues please post your question in the Adobe Photoshop Elements Forum. Somehow your
question got posted in the Adobe Premiere Elements Forum.
b. Have you verified your Photoshop Elements serial number at the Adobe web site (http://www.adobe.com) - after signing in with your
Adobe ID and Password, type My Products in the Search field and follow the path to your product in My Products. Click on Adobe
Photoshop Elements 12 in your list to reveal the serial number.
c. Do the install with antivirus and firewall(s) disabled.