Since this is a 2011/2013 title, the most reliable and legal way to download it for PC is through major digital storefronts:
Another legitimate key provider that often supports charity with your purchase. Why Play XXL? Download Serious SAM Double D XXL PC Game 2011
Frequently carries Serious Sam titles DRM-free, which is great if you want to own the installer files directly. Since this is a 2011/2013 title, the most
Fight bizarre foes like "Vuvuzela Drones" and "Chimpanzees on Unicycles" across 21 levels [1]. Since this is a 2011/2013 title