Warning: Be careful when editing the registry!
This article shows you how to add an entry in the right click menu to "Open in Notepad". Its quite easy to change that for emacs. Just change the notepad.exe %1 to:
C:\Program Files\emacs\bin\runemacs.exe "%1" and that should work.
Their is one problem with this. Each time you do this, a new emacs window will open. I'm sure their is a way to not let that happen, I just haven't had time to deal with it on windows yet.