Okay, I think the problem is, indeed, with my credit card. On my PC, I've successfully downloaded several of the books from my library, but those were, I guess, DRM-free. A few of my books don't download, saying that my credit card is invalid. I though B&N accepted non-US credit cards? Why is it saying mine is invalid?
Thanks for the link graphicsgirl. Unfortunately, that didn't quite work. In fact, in the article the guy said:
Once the battery is removed, connect your NOOK to a wall outlet using the NOOK AC-Adapter. The NOOK should begin to go through its booting process. Once booted, replace the battery (don't turn the NOOK off or unplug) and charge your NOOK for at least 2 hours.
Well, in my case, when I remove the battery and then plug in the power cable, nothing happens. The thing doesn't boot up. I had to plug the battery back in.