Google search with 'default email client from registry' returns link
http://www.xs4all.nl/~nlvanloo/win98_defaultemailclient.html

line on this page:
To set the default email client update the following registry key:
HKEY_CLASSES_ROOT\mailto\shell\open\command

Another good link from the search above:
Q. How can I set the default email program?
http://www.ntfaq.com/Articles/Index.cfm?ArticleID=26808