well i know Word comes with a math editor. but if you want something very professional looking, i strongly recommend you take the time to learn LaTeX (a typesetting language). this will allow you to write math in any text editor and just compile it to .dvi, .ps, .pdf, etc. yeah, there's a learning curve, but it's powerful.
look here:
http://www.latex-project.org/
and here:
http://www.miktex.org/
for a comprehensive and free install.