Where did the install file come from? Is it the original DVD? If yes, try downloading from here
or here
Prodesign Tools — Photoshop Direct Download Links
How many products do you have listed in your Adobe account?
Is your Photoshop CS5 a stand alone product, or part of Creative Suite?
Are you trying to install on the same computer platform? (Windows or Mac — your serial number will only work on the same platform it was purchased for.)