I think this is crappy software and severly limits portability options. I was able to print the free stuff, but not what I spent hard earned money for. A load of nonsense.
Jamie, when you downloaded the book, was the file your PC attempted to open something like ebx.etd or something like URLLink.acsm? The file extension (etd/acsm) is the important bit.
etd files are sent by the old version of the Adobe Content Server (version 3) and acsm files are sent by the new version (version 4). These files are keys to your book, not the book itself. The book is downloaded once the key has been validated.
As of early May 2009, Adobe Digital Editions will no longer open etd files.
If you were sent an etd file, read on, otherwise this problem may be something else better answered by Adobe support channels.
If you still have Adobe Reader version 6 or 7 installed on your machine (and it was installed prior to April 2009), you could try opening the etd file with that instead. Versions 8 and 9 will not work with Adobe Content Server (any version) files I'm afraid.
Otherwise your best course of action is to go back to the supplier and ask them for a new document download, but this time ensure they are sending the download from Content Server 4.