To do so in Windows:
- Download the Gmail Notifier.
- Right-click the Notifier icon in your system tray, and select Options.
- Check the box next to Use Gmail for internet mailto: links.
- Click OK.
And, if you're using a Mac, you can set your default mailto handler by following these steps:
- Download the Google Notifier.
- Open the Notifier.
- Click the Mail icon on the Menu bar and go to Preferences...
- Select the Gmail tab and choose Gmail as the program from the Compose mail in drop-down menu.