Its probably because of a "bad" file in one of the folders you are trying to import. Matt was kind enough a few builds back to add in a whole bunch of logging functions which should help you track down which file is causing it to die. Find the log folder inside your Library (which if you didn't modify it defaults to C:\Documents and Settings\[Windows_UserName]\Application Data\J River\Media Center 11\Library)... Then open the most recent Import Log file and under where it says:
Importing new files (3/16/2005 11:01:38 PM)
Find the last file listed (it will probably be the last line in the file if MC is crashing). The offending file is probably that one, or one of them right around it (either before or after the file in an alphabetical list). Typically when this happens to me (not often, but I have had a few corrupt files here and there before I fixed my hard drive situation), I find the file is messed up anyway (0 bytes and/or won't play in anything). Either way, delete the file or move it out of MC's search path (you may have to do this from the command line in Windows if it crashes Explorer when you select the file), and you should be all set.