In this period of Gmail diffusion, lots of people have tryed to create a tool to be notified when an email is received on their Gmail accont.
Now, Google has provided us with the first official Gmail Notifier (now it's on beta stage).
The Gmail Notifier is a downloadable Windows application that alerts you when you have new Gmail messages. It displays an icon in your system tray to let you know if you have unread Gmail messages, and shows you their subjects, senders and snippets, all without your having to open a web browser. You can play a sound when an email is received and also it has the ability to connect a mailto: address on a web page to your Gmail account.
The tool that I've been waiting for...
P.S. It checks for email availability every 2 minutes... I hope that on next releases this time could be set as you want.