Problem is usually with extensions.
Mac OS X, Linux etc are usually smart enough to know what program should open a file even if it doesn't have an extension (i.e. word docs end in .doc).
Sometimes Word for Mac doesn't save extensions to files. If a windows user receives the file windows doesn't know what to do with it. They can easily just add a .doc to it and it should open.
I've never used mail's "windows-friendly" but always made sure that every file I sent via email had the proper extension. Easiest way to make sure if select the file in finder and hit apple-i (or right-click and choose 'get info'

, under 'name & extension' make sure that there is an extension (i.e. .doc) on the file. Then just send via email.
I'm a computer lab admin at Massey and have to send files a lot and have never had a problems with windows users since always checking to make sure there is the proper extensions.
Let me know how it goes.
Jay.