Depending on the complexity of the password a brute force algorithm that runs though all the possibilities is likely to take years, if not tens or hundreds of years, depending on the amount and type of resources you can dedicate to the job. If you at least remember part of the password or the characters used you can narrow it down and speed up the process, but that's about it. There's no known backdoor in any of the recent RAR versions so if you can't brute force the password in a reasonable amount of time you're simply out of luck.
Edit: If the program you're talking about is this one, all it does is convert locked (i.e. read-only) RAR archives to read-write mode. Locked RARs are not the same as password protected RARs.