You need to download decoding software for it. Install winrar: http://www.rarlab.com/ ,then you should be able to open the file. It's free for thirty days, after which it expires-which means you'll just get a popup every time you open a file through winrar asking you to register is all-it will still be fully functional.
If you have the compression program winrar already installed, right click the file and select "extract to" and send it to a folder you create for it somewhere easy to find. I like to keep a folder directly under my C drive called 0temp. Putting the number 0 at the front of the name assures it will be the first directory under the C drive. You can also configure your downloader to use that folder instead of using the desktop, which will eventually get filled up with rar's and files if you continue to put them there.