Neither Gmail Notifier nor Google Talk nor Google Toolbar works consistently to make Gmail the default email client. GoogleCity » GMail Default Email Program / Start Menu looks promising, but than adding "Google Gmail" in the Internet options, it doesn't work.
Hack: Window Registry Edit to make Gmail the default mail handler should work but the file paths are not updated, so I have corrected and optimized it. Somehow it seems that a local file (dll / exe) is needed to direct email requests to a web mail and in this case gnotify.exe of Google Notifier is needed. If Google Notifier is installed, simply download this registry file and merge by opening it. Change the paths before merging if Firefox or Google Notifier are not in their default directories, or if Firefox is not used.
It should work for the default browser "send link", browser initiated email compose, mailto links and commands, plus change the email icon in Windows XP start menu to Gmail (this can be removed afterwards). "Google Gmail" is chosen in the Internet options, and emails are directed to the more secure https Gmail.