How change Windows 7 so applications launch Thunderbird instead of Outlook?

Question

I have a number of applications (e.g. GoToMeeing) which open up an e-mail which is ready to write. However, they always open Outlook instead of the e-mail client that I use: Thunderbird.

Where is the config file / setting in Windows7 so applications open Thunderbird by default instead of Outlook?

Addendum

Thunderbird is not in my "list of standard programs":

Answer 1

• Click the Start Menu and select Default Programs from the right hand menu column.
• From there, click Default Program again.
• Highlight Thunderbird from the list that gets built and click Choose defaults for this program.
• Finally, select the mail option from the list of actions and click Save.

If the program is not in the list of possible, try this:

• Back up one screen to the Default programs page
• Select Associate a file type or protocol with a program
• Scroll to the very bottom of the list to the protocol section
• Select the program you want to use or Browse to find Thunderbird.