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Could you open your Mac hard drive and go to Applications/Utilities/Adobe Installers.
Let me know if you can find the add or remove photoshop application.
If not could you go to http://www.adobe.com/support/contact/cs3clean.html then download and runt he CS3 cleaner script.
After you have uninstalled could ou reboot then try to install again.
Let us know what happens.