If you're experiencing any download problems, please redownload with the GOG Downloader. To do this, make sure the "GOG.com Downloader mode" is switched to ON before clicking the "Game installer" button.
The new GOG.com Downloader now features full error-correction functionality and thus should be able to cope with any download corruption issues, as well as offering the fastest possible download speed. Most importantly, it can pause and resume downloads, so it's perfect for large downloads and unstable Internet connections.
If you experience problems with the GOG.com Downloader, go here.
If your installer is corrupt even after downloading through the Downloader, you should check your RAM and hard drive for errors. For details on how to perform these checks, go here.