Newsrover will do that for you. All you do is right click on any single rar, and choose "Download and decompress RAR archive". If PAR files are present and some RARs are missing, Newsrover will automatically recreate the missing RARs, then unrar them to your download directory automatically.

Plus, if you subscribe to a news provider that has multiple servers, or if you have access to multiple servers, you can set their priorities. So, lets say a few articles are missing from a multi-part binary post on your primary server, Newsrover will search each server until it can find those matching articles on another server. Its pretty cool.

PAR files kick ass, I was so happy to see those pop up.