You can crack it using a brute force or password list. A two character password can be cracked in around 15 seconds; however a 8 character password with a special character can literally take years. I tried for 6 months to crack a password on a rar file, using around 16 high powered servers and got nowhere....
in reality, no... it's not worth it. Either find the original password or delete the file.
it is a waste of time to try and crack the file.
To find the original password... go back to the site where you downloaded the file from and see if someone posted the password.
Or google for the file name and "password"
BTW... if it is a P2P file that you are trying to open there is a strong likelihood that the file is fake.
Normal P2P releases are rarely password protected
Like the other people have said, it's most likely you cannot recover the password on that RAR file.
You might need to go to the original site you got it and see if the poster also posted the password for that file, most likely, it's the site's name or the name of the poster.