Rich - if it's coming up with a CRC checker then re-download the torrent and unselect all the rar parts except for the one that's corrupt. Make sure you save it into a different folder - this sometimes happens to me when I have problems with the computer at my end (or if I am using a USB hard drive for that torrent and it resets itself).

Once the rar file has downloaded again, copy it over the corrupt one in the original folder. Should be job's a good'un, if you're using the same site I'm using (which I think you will be) then I can verify that the file is fine and it must have just been a bad seeder on your download or a problem on your comp.

Cheers,

Andy