Microsoft Office 2011 OS X and unicode
If the problem is that some characters do not appear but small rectangles are shown instead, as I suspect, then it’s probably a font problem. Quite possibly the Windows machines lack fonts that contain the characters needed.

In that case, you could download and install some suitable free fonts on the Windows machines. To identify a character (possibly displayed as a small rectangle) in a Windows version of Word, click on the position right after the character and press Alt X. This turns the character to its Unicode number. For example, ∅ turns to 2300. The Unicode number can then be used at http://www.fileformat.info/info/unicode/char/search.htm where you can find information on font support to different characters, using e.g. character name or Unicode number as key.