how to find rar pw?

i have a rar that has a pw on it. how do i find the pw?
lol the person didnt give anything..

btw what happend here?

http://www.rarpasswordcracker.com/

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Version 4.12 (zip, 201 Kb):

* Site #1: http://www.rarpasswordcracker.com/rpc412.zip

Version 4.12 (exe, 205 Kb):

* Site #1: http://www.rarpasswordcracker.com/rpc412_setup.exe