First of all, you need to download Mozilla Thunderbird setup file from its official page.
For that purpose, you need to open your web browser and write “mozilla.org/thunderbird”, without the quotes, into your web browser’s address bar and press the “Enter” key on your keyboard.
On the main page, you will see a big green button with “Thunderbird Free Download” written in the center of it. You have to click on it, in order to download the setup file to your computer hard drive.
If you are using Chrome or Firefox to download the program, then it will be saved in your default download folder. If you are using Internet Explorer as your web browser, then you will have to define a location where you want to save the downloaded file.
Once the download process is complete, go to the folder where the file is saved and double click on it to start the installation process.
As you are using Windows Vista to install Mozilla Thunderbird, it will ask your confirmation to install the program in your computer. Click “Yes” to proceed.
Click the “Next” button twice and follow the onscreen prompts to complete the installation process. During the installation, you need to make sure that you select “I Agree” when the “Terms and Agreements” page appears, or else, the installation process will be aborted.
Once the installation process is complete, click the “Finish” button, and double click the program icon on your desktop to launch the program. When launched for the first time, you will be asked to whether import your contacts directly from MS Outlook, if you already have it installed on your computer. Follow the onscreen prompts to add email, RSS and newsgroups to Thunderbird.