G-Mail is not listed under the E-mail link: in the properties of the Start task bar.
1.) Choose an Icon (*.ico file) to use for the E-Mail link in the Start Menu -- I downloaded one from here and rename it to Gmail-stamp.ico
2.) Place the Icon in your Windows Folder (usually C:\Windows\)
3.) Download and install Google Notifier from here (its up to you if you choose to run it at startup -- you dont have to)
4.) Download GmailVista.reg and import it into the registry
5.) Right click the clock in the system notification area and choose properties. Navigate to the Start Menu tab and click Customize under the Show on Start Menu section select G-Mail from the E-mail link: list.
To restore Windows Mail as the default Client you can use DeleteGmailVista.reg