This is a repost from http://www.portalprelude.com/
This is the download labeled as Portal: Prelude Installer (Microsoft Windows) (1.1.5). This is the base executable for Microsoft Windows.
It will extract all the files at the right place automatically.

About Portal: Prelude

Why ?
In January 2008, I was searching for a new project. Portal was out since four months, and still, no-one had already did something really serious around it. I was following a lot a communities, seeing a lot of independant releases, but no mod and no serious story arc. This I why I decided to launch this project.

How ?
At start, the project was rather insignificant. It was not supposed to be a mod (modification of a game), but just a succession of test chambers, without any real logic between each of them. I had no goal, I was just making maps as I saw it. And finally, when I reached a dozen of test chambers, I realized that I would be a shame to waste all my work by only releasing yet another map pack without any real story. This is where I began to write a whole new story and decided to follow the famous belief that prequels are somewhat popular recently.

What ?
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...

Who ?
My goal was to make it a personnal project, without telling anyone about it. I wanted to surprise everyone by releasing a quality, polished and fully finished game, offering roughly the same amount of playtime than Portal. As months passed, I had more and more difficulties to keep working regularly on it. In real life, I'm both a student and an employee, and that didn't left me a lot of time to work on the game. This is where I decided to ask for some help, mainly from Jérémy "Capitaine Mousse" Barbe, who helped me by making four of the nineteen test chambers while I was working on the final level. Marc "moroes" Bidoul helped me too, by giving out his level-designer point of view and making a few 3D models and textures.

How long ?
Nine months. Not nine month of continuous work of course, but nine month with some days filled with dozens of hours of hard work. I worked on the nineteen test chambers for 3 to 4 months, followed by 2 months on the final level, 2 months of tests, debug, and polishing (interfaces, voices, commentaries, etc.) and an extra month for communication stuff and the preparation of the release. I must confess that it took me a lot more time than I initially thought, mainly because I had exams and work to do on behalf.

And after ?
In June 2008, I learned as everyone else that Valve was working on an official prequel of Portal. I must say that I was mostly the only guy on Earth not really happy with this announcement. So, I decided to work twice as hard to release my version way before Valve's one. I hope that they will not take it too personnally, because mine was in development 5 months before their first unofficial announcement.