First follow this link and download the Gmail Notifer...
I know, now you're thinking there is going to be another start up program in your tray... but don't worry, we'll fix that.
Next, run the program and uncheck the "Run on startup" box:
Then enter your password:
Finally, right click the mail icon at the bottom right-hand corner of your screen and click exit:
That is it! Now go find all the Mailto: links you can find and be amazed!
Note: The gmail app will log on to whoever used gmail last. So if you have more than one gmail account, make sure you are sending it from the right account! This can be done by visiting mail.google.com and noting what account shows up.
For Mac users it is very similar:
1) Download the Google Notifier and run it.
2) Go to Preferences by Click the Mail icon on the Menu bar
3) Select the Gmail tab
4)Uncheck the "Start on Login" and "Display Message Count" option, and choose Gmail as the program from the "Default Email Program" drop-down menu.
5) Enjoy the convenience of Gmail as your default app!