Just burn out the files to a CD or move them to an external HDD or something first, and move it back over to the download folder once you've formated your computer. When you den click the download again on the site or - if you keep the .torrent file - you could just start it again with that, it should read thru the files you've allready got and start up from where you ended it.
And, not that you would have any need to do so, but don't rename the files and folder of the download or anything.
And, not that you would have any need to do so, but don't rename the files and folder of the download or anything.