Microsoft is releasing Windows 7 beta to everybody who wants it this afternoon. Developers have already had access to it for a couple of days.
You will be able to download an .ISO image of Windows 7 beta that you have to burn to a DVD in order to run the install. If you do not know how to do that, you should not install beta operating systems anyway.
The hurdles to install Windows 7 beta do not stop at the ISO image. You actually need to have a Windows Vista SP1 running that you can upgrade with the Windows 7 beta.
You cannot upgrade XP or do a clean install with this release of Windows 7 beta. Microsoft gives you though the choice of downloading the 32-bit or the 64-bit version of Windows 7.
Lastly the Windows 7 beta expires August 1st. So do not even think about using it for real work or storing your data under the beta. Info about Windows 7 beta is via the Windows Team blog. Update: Windows 7 Beta download started

The Windows 7 beta download is supposed to start this afternoon on the Windows 7 page and on Windows.com. Microsoft will only serve up 2.5m downloads of Windows 7 beta, but because its an ISO file there sure will be copies available on filesharing services and download sites.