Verified answer

Like Windows XP, you can access the Default Programs applet in Windows Vista from Internet Explorer: go under Tools > Options, and the Internet Options dialog will open; click on the Programs tab, and click the Set Programs button. This will launch Windows Vista's Default Programs window: click on the link which reads "Set your default programs", right below Choose the programs that Windows uses by default.

OR

http://www.williamrandol.com/GmailIconNotifier.htm

It should ask you when you start it up or it will be under options/preferences, but if not, look for Default Programs or something like that.