There are other people with the same problem. They are posting on this very same forum (Debian). Check out the other threads (they have "811" in the title of the thread). As I suggested in one of them, when you get to the point in the installation where you check off what software you want (mail server, file server, etc.) uncheck everything and don't check anything and see if the installation proceeds normally. It shouldn't take long cause you won't have too many packages. If it does work you can always add stuff later.
As I said, you should check the other threads on the same subject in this forum going on right now.