"Error writing file"
This error message most commonly indicates that the drive on which you're attempting to install your Garritan library is full. Try uninstalling unneeded software to make room for the Garritan samples.
"Cannot find compressed samples"
This error message means that the installer could not find the SamplesData folder. This folder must exist in the same location as the installer itself. If you have dragged the installer out of the unzipped folder, please redownlad the installer and run it from the original directory.