This error message indicates that there is a problem with the database of Picasa and the easiest way to fix it is to actually delete the database. You will not lose all your photos after this operation, but the simplest fix is to uninstall the product. Go to Control panel > Uninstall a program and select Picasa. Remove it and then reboot the computer. Download a new copy through the application's page at Software Informer and install it then rebuild the database and that's it.