If you are looking to download Windows 7 Beta right now its possible directly from the official Microsoft website because its available to the first 2.5 million users who want to download it.
Here are the 2 websites from which you can download the ISO Images which you can later burn and then install after getting them activated using the product keys.
The normal installation would work out for 90 days and to extend this time frame you need to activate, after which the windows 7 licence would work out till August 2009, pretty good to get your hands on the OS. Even if the download links dont work out in the initial keep refreshing the website and finally you can get it working, because the downloads are currently done heavily.
Download Links :
Windows 7 Beta (32-bit)
Windows 7 Beta (64-bit)