The tutorial explains the problem well. Unfortunately the only remedy given for Windows users is a program called WMDecoder which has a 30-day limit (unless you pay for it).
As an alternative, there is a web page that users can upload the winmail.dat file to for decoding:
http://tud.at/php/tnef/
More of a hassle than an on-disk program, and limited to files of 4 megs or less, but free.