cannot send attachment

I have a new computer withe vista and currently useing the free trial version of office 2007 that came with it. When trying to send a attachment in windows mial in any formatt I get a popup that says: can not send due to missing or can find some or all files. My wife is taking online classes and has to send her home work in a rich text formatt so that the instructor can open and read it. I have tried every formatt possible and still have the same problem, that it want send the attachment. Please some body help me.....