try downloading an applaccation like appzapper which deletes everything to do with the program, i use appzaper alot its really useful.
download appzapper Here
Assuming you have downloaded a file that has been broken into parts, say part1.rar through to part6.rar, create a new folder and move all the parts of the file into this new folder. Now open UnrarX, click on "Browse" in the upper right hand corner, and navigate to the folder you moved all the parts of the file into, select part1.rar and press enter. Back in UnrarX now click on "Extract" which is next to the "Browse" button and let UnrarX do its magic.
UnRarX will update you with progress in the form of a text log as it uncompresses and stitches all files together, this can take a while depending on the size of the the files you are working with so be patient. Upon finish, you will see at the bottom of the text log "All files Okay......." now you can quit UnRarX and go back into the folder where you put the part files, you will now find the resulting file from all the parts.
Same answer (or just drag the rar file onto unrarx's icon). If it doesn't work and unrarx give you error messages, the rar file is probably corrupt.
Try re-downloading it.
Like MacBurg said, you should just download the newest version of StuffIt Expander. It can open RAR files, as well as other types of archives. Here's the site.
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