To use existing instance of Emacs : First take a look at GnuClient.
Add the following key to the registry:
HKEY_CLASSES_ROOT/*/shell/Open with Emacs/command
with the value (of type REG_SZ)
c:\emacs\emacs-20.7\bin\gnuclientw.exe "%1"
On W2000 save and edit the following registry file and install it:
--snip--emacs.reg-- [HKEY_CLASSES_ROOT\*\shell] @="*" [HKEY_CLASSES_ROOT\*\shell\emacs] @="Open with Emacs ..." [HKEY_CLASSES_ROOT\*\shell\emacs\command] @="PATH\\TO\\GNUSERV\\gnuclientw.exe \"%1\"" --snap--emacs.reg--
On Windows XP it looks like one has to specify that it is a regedit4 script
--snip--emacs.reg-- REGEDIT4 [HKEY_CLASSES_ROOT\*\shell] @="*" [HKEY_CLASSES_ROOT\*\shell\emacs] @="Open with Emacs ..." [HKEY_CLASSES_ROOT\*\shell\emacs\command] @="PATH\\TO\\GNUSERV\\gnuclientw.exe \"%1\"" --snap--emacs.reg--