How did you get the .rar files on the phone? Is it jailbroken?
Anyway, I don't know of such software for iOS, maybe it would be easier to get those files onto your Mac OS X or Windows or Linux computer and use the respective applications to decompress those files and get the resulting files onto your phone again. Or don't you have access to a working computer? OS does not really matter, as the ones I mentioned have the ability and the applications to handle those files.
Btw:
"How to maximise your MacRumors troubleshooting experience" created by mad jew in 2006