You don't, once you've download the rar file from bittorrent you have to also download winrar to decompress the files.
http://www.rarlab.com/download.htm
Just be sure you download the correct version for your windows. Don't worry bout the fact it says trail version. Its got no expiration timer, it just tells you that you need to uninstall after 40 days. But still allows you to use the thing.