Portal: Prelude, as its name states it, is an unofficial prequel to the game Portal. Its story revolves around the pre-GlaDOS epoch, even before she was plugged in. At this time, test subjects were monitored by real Aperture Science employees whose work was tedious, lengthy and repetitive. This is why they decided to build a great artificial intelligence that could both replace them in these difficult tasks, but also take responsibility for many other tasks within the complex and compete with Black Mesa's superiority. All employees of the Aperture Science complex are now eagerly awaiting GlaDOS. Maybe even a little too eagerly, as the upcoming events will tell...
Portal: Prelude is free, but requires the full legit retail version of Portal (including Steam) in order to install and play it. This torrent is simply the DiGiTALZoNE release of the original Steam-cracked Portal, with the Prelude mod dumped into the game folder, turning it into a standalone version of Portal: Prelude.
This torrent simply saves you from having to:
..apply the mod yourself
..remember your Steam login and password, if you forgot it
..potentially scratch your original Portal disc(s)
..have an Internet connection in order to play the game
..have your game usage tracked by third-party entities
Please let me know if it doesn't work for you. It should work fine, just launch PORTAL.EXE to play it.