To install Microsoft Equation Editor:
http://www.wellesley.edu/Computing/WinXP/Equation_Editor/equation_editor.html
Then, to write the equation, do what bos1234 said:
Microsoft Word > Insert > Object > Microsoft Equation 3.0
I've attached a document containing the keyboard shortcuts. Once you learn these, it becomes quite fast to type out an equation.
Then, to get the equation onto the forum:
1. Press the "Print Screen" button on your keyboard
2. Open Paint
3. Paste in the screenshot
4. Select the equation using the rectangle selecting tool, then edit>cut
5. Create a new Paint file, paste in the equation that you cut out
6. Make sure you crop the edges (drag the corners)
7. save the file as a "GIF"
8. go to
www.imageshack.us and upload the GIF
9. in bos, type [noparse]
(where url is the direct link to the image)[/noparse]
It's a bit slow at first but once you've done it a few times it won't take any time at all.
Hope this helps!