I was having the same problem, but I am now able to read my ebook. Note: I tried several different things, so I'm not exactly sure which one fixed my issue, but at least I'm able to read my book and didn't waste the money. Here are the steps that I took; I hope they will work for you as well.
1. Earlier today I downloaded the latest version of ADE (v4.5). The download was not problematic, but I couldn't open the ebook that I had bought. Every time I click on the thumbnail, the application would freeze. I was able to open the Getting Started book that came with Adobe.
2. This evening, I uninstalled ADE 4.5
3. I downloaded ADE 3.0. During the install, I was prompted that I needed to download a Microsoft .Net service pack. So I did. Then I completed the ADE 3.0 download.
4. Once ADE 3.0 was installed, I was still having issues opening my ebook. The system was not freezing up, but I was getting an error message. I don't remember the exact error message, but it was a Activation Error: Adept_Internal, or something like that.
5. So I tried reinstalling ADE 4.5.6. ADE 4.5 continued to freeze and ADE 3.0 continued to have the error message
6. Next I went to the actual file and double clicked on it. The file can be found at My Documents > My digital Editions.
7. I didn't get the error message, but I got a message saying my computer (or book, can't remember which one) needed a Vendor ID. I clicked continue and entered my Adobe ID and password. I then got a message that my computer was authorized (even though I had authorized it earlier today when I first downloaded ADE 4.5)
Note: I don't know what it means to have "Vendor ID", so if you don't feel comfortable doing that you probably shouldn't. Also, I don't think step 5 above was necessary because the book was opened in ADE 3.0.