It could be that the downloaded file is corrupt, or that your anti-virus software is causing the error.
If you are running the latest version of Silhouette Studio (with 'Cloud' support), then the Library Extractor will not extract the library files at the moment (the 'Cloud' library is in a different format).
If you are not using the 'Cloud' version, backup your library to an external drive or memory stick so you can transfer them to your new computer.
Ian