Windows is giving out a free copy of windows 7 for the beta testers and I wanted to know how all this works.
You can download the Windows 7 Ultimate release candidate directly from microsoft. You burn the ISO file to a CD and install it. They give you a product key and it works until about March or so of next year, after which you need to buy it or get rid of it.

They don't recommend installing it on your main PC. It works very well, but you don't generally want to be installing and uninstalling the OS on a PC you need critically.

http://www.microsoft.com/windows/windows-7/get/download.aspx
You download it and burn it to a disk and either format your computer and do a fresh install of the release candidate or make a partition on your hard drive and install it on there. Windows 7 is actually finished it's not the RC anymore it's the RTM (release to manufacturer). You can have it installed on your computer until March 2010 and it'll then begin shutting down every two hours unless you buy a legit copy of the OS. If you know how to use torrent sites you can actually download the finished copy of Windows 7 now and crack it and have the full version forever, but that's all up to you.
The only people who will be receiving the free copy of Windows 7 are the beta testers that were handpicked by Microsoft themselves to be in a closed beta, they are not giving one to the people who were in the open beta that could be entered by anyone. So unless you received an email about a year ago stating that you were chosen for a closed beta of Windows 7 you won't be receiving the free copy.
It's easy to get the RC, simply go to this website http://www.microsoft.com/windows/windows-7/get/download.aspx then scroll down, click either 32 or 64 bit depending on what you need, then it will get you to log into your windows live account. You can begin downloading an ISO file of the operating system. I got the 64 bit version and it was 3.4GB if memory serves.

Once it downloads, burn the ISO onto a disc, if you don't have a program capable of this download a program that can open ISO files and burn it. Put the disc into your PC and reboot, make sure in your BIOS that cd/dvd is first in the boot priority list and your computer will tell you to hit any key to boot from the disc. Follow the on-screen instructions and you'll have windows 7!

Good luck!
You had to be part of the invitation-only Technical Preview program.

If you were just a member of the public who downloaded the beta and "tested" it you're not eligible.
Donload it from http://microsoft.com downloads as an iso file which you then need to burn to a DVD which then becomes your installation disk. You only have it for a year which is why it is recommended you pu it on an empty spare partition rather than having it your sole operaing system. March 2010 it will start shutting down every two hours and by June it will be gone completely
its free to January 2010 then you will have to pay for it, however because you been a "tester" you may get it cheap.