I used to have the nook for PC app installed on my computer but I don't even do that anymore. As somebody up there said, you just buy the book and put it in your library, then when you click on download it just saves a file to your hard drive.
The Calibre plug-in for Barnes and Noble will remove the DRM. If you Google "Apprentice Alf Blog" and look at the entry for tools v2.3 you will find it. Then it is just a simple matter of converting from ePub to Mobi.