Suggestion: Check under "URL/mailto:" (or something like that) in the File Types dialog in Explorer.

Yup -- I found "URL:MailTo Protocol" there, and searching for that term in RegEdit brought me to:

Windows Registry Editor Version 5.00

[HKEY_CLASSES_ROOT\\mailto]
@="URL:MailTo Protocol"
"EditFlags"=hex:02,00,00,00
"URL Protocol"=""

[HKEY_CLASSES_ROOT\\mailto\\DefaultIcon]
@="\\"\\MICROS~2\\Office\\OUTLOOK.EXE\\",7"

[HKEY_CLASSES_ROOT\\mailto\\shell]

[HKEY_CLASSES_ROOT\\mailto\\shell\\open]

[HKEY_CLASSES_ROOT\\mailto\\shell\\open\\command]
@="\\"\\MICROS~2\\Office\\OUTLOOK.EXE\\" -c IPM.Note /m \\"%1\\""

The actual "\\shell\\open" command is probably what you are after, to change to whatever app you want to use.

(And, hey, don't blame *me* for the one above -- this is a job machine, with our Corporate Standard setup! :-)