Download
gmail notifier and during the installation it will ask if you want gmail to be your default email program.
from the
gmail FAQ:
How do I make Gmail my default mail application?
You can designate Gmail as your default mailto handler. This means that when you click on any hyperlinked email address, a Gmail compose window opens so you can send a message to that address, from Gmail.
This feature is turned off by default. To turn this feature on after downloading the Notifier:
1. Choose ?I Agree? to the License Agreement
2. Check the ?Use for outgoing mail? box and click ?Next.?
3. Click ?Install.?
To change this setting, you will need to download the Notifier again and make your change on the ?Choose Components? screen.