Application Manager prompt while trying to open Photoshop CS5 Extended

Hi,
I have a problem since yesterday. When I try to open Photoshop I get such a prompt:

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Adobe Photoshop CS5 Extended

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Adobe Application Manager is needed to start your trial.

Adobe Application Manager is needed to resolve this problem. However, it is missing or damaged.

Download a new copy of Adobe Application Manager from http://www.adobe.com,

or install this product again.

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OK  

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For some reason it doesn't see my serial number. So I installed Adobe Application Manager and it opens Photoshop in trial mode asking to enter my serial number or start a 30-day trial. I don't have my CD or box any more. Is my serial number stored anywhere on my computer (Windows 7, 64-bit)?
I haven't been reinstalling Photoshop. It has been running fine and this issue started to happen suddenly.

Please help.