Using Backup and Migrate Module to download windows compatible backup
There's a couple steps you still need to do:


To "untar" the tar ball on Windows, you just need the right software for it. An easy to use and free software that supports TONS of compression types is 7-Zip -- http://www.filehippo.com/download_7-zip_64.
Once you have all the files available to you, you'll need to import the database into MySQL. Luckily WampServer comes with phpMyAdmin. You can find a link to it in the tray icon for WampServer. First create your database, then use the Import tab to load in the SQL file.
Modify the settings.php file to match the database database connection information for your local MySQL installation. The default installation of Drupal creates this file is at /sites/default/settings.php.
Then visit http:// my-local-host/ (or http:// my-local-host/[my-drupal-subdirectory]/) to see if it's working. (Replace "my-local-host" with "localhost" since StackExchange doesn't like "localhost" in a link).