完整答案，来自MSDN
------------------------------
When you select Make this the default browser in Internet Explorer and later
versions of Netscape, the browser registers itself into all of the following
keys to make it the "default" browser (Even though more entries are being
written).
HKEY_CLASSES_ROOT\.htm
HKEY_CLASSES_ROOT\.html
HKEY_CLASSES_ROOT\http\shell\open\command
HKEY_CLASSES_ROOT\http\shell\open\ddeexec\Application
HKEY_CLASSES_ROOT\ftp\shell\open\command
HKEY_CLASSES_ROOT\ftp\shell\open\ddeexec\Application
HKEY_CLASSES_ROOT\gopher\shell\open\command
HKEY_CLASSES_ROOT\gopher\shell\open\ddeexec\Application