How can you work on a Microsoft Word 2007 document with someone else via the internet?

Well, my buddy has Microsoft Word 2003 and is working on a paper. I have MS Word 2007 and would like to help him write it.. is there a way this can be done? I know it can be done with an 03-to-03 but can an 07 work with an 03 at the same time?

How can you work on a Microsoft Word 2007 document with someone else via the internet?
do you mean edit at real time (never knew office can do that, wow)


or do you mean capability between the different office, if that, make sure to save it in doc, do not use docx