How to uninstall iPhoto so I can reinstall it from the App Store

My school upgraded my OS to Lion, and when they installed it, they installed iLife 11. Everything works except iPhoto, which refuses to work, even though everything has been updated using Software Update. I now want to buy iPhoto from the App Store but every time I do this I get this message:

Could Not Complete Your Purchase
iPhoto is already installed on this computer. Choose Software Update from the Apple menu to check for available updates.

Now I have placed iPhoto in the trash and emptied it, and I have also removed all the PKG files from Receipts in the Library, but it still refuses to work. I have also looked, and iPhoto is NOT under purchases in my App Store so I can't uninstall it from there.

Does anyone have any suggestions on how to fix this problem?

Much Appreciated

Personally I would get your school to fix this. They messed it up - they should fix it.

Does anyone have any suggestions on how to fix this problem?

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It's very likely that you have not completely removed iPhoto and that's why it refuses to download from the App store. Do a Spotlight search and if necessary download the free "Find Any File" and use it to search with.