You need to make a backup of your Thunderbird Profile folder and store it on an external drive or usb memory stick etc. Make hidden files and folders visible: * http://kb.mozillazine.org/Show_hidden_files_and_folders Easy to locate via Thunderbird. In Thunderbird: Help > TRoubleshooting Information click on 'show folder' button It will open a new window showing you the contents of your Profile folder name. '''Close thunderbird now - this is important.''' Your profilefolder name will usually be xxxxxxxx.default, where the x's are letters and numbers. Go up the directory and copy the xxxxxxxx.default folder. Save to external device. More info: * http://kb.mozillazine.org/Profile_folder_-_Thunderbird After you have reinstalled Windows OS. Download and install Thunderbird : * https://www.mozilla.org/en-US/thunderbird/all/ Run Thunderbird the first time and it will create a new default profile folder name. It will ask you to create a mail account - do not create any mail accounts. Access the profile folder name as before; you may need to make hidden files visible. Help > TRoubleshooting Information click on 'show folder' button It will open a new window showing you the contents of your new xxxxxxx.default Profile folder name. '''Close thunderbird now - this is important.''' These are newly created default files and folders. You need to access your backup xxxxxxxx.default file. Copy '''all''' the '''contents''' of the backup xxxxxxxx.default file and then paste them into the new xxxxxxxx.default file overwritting all the files and folders. As an aid. so you know you are looking in the right place, I include an image below, which shows an example of the contents of a xxxxxxx.default Profile folder name. These are the files and folders you are copy pasting from backup profile folder name into the new profile folder name. Then start Thunderbird.