Before you download Adobe Digital Editions, you want to make sure you download version 2.0.1. The reason for that is because newer versions of ADE have the ability to use a different DRM and this newer DRM means that there are situations where you'd not be able to read eBooks with this new DRM other than on your computer using ADE.
Windows:
http://download.adobe.com/pub/adobe/..._Installer.exe
Macintosh:
http://download.adobe.com/pub/adobe/..._Installer.dmg