You can officially download from here :
http://www.microsoft.com/windows/windows-7/download.aspx
If you are using XP, you will love it.
If you are using Vista, i am sure you are going to stick on it.
Anyway, it is still RC (Release Candidate) version. Still got some bugs and some changes in future.
Try it, and you will love it.
What? Your PC is running Pentium 3? No worry, it’s works!