ARS itself is really easy to install :) but some of its Prerequisites are not :( , depending on which operating system is used. The best option is Ubuntu, specially its 12.04 (i.e. Precise Pangolin) version. For that, follow the instructions in Ubuntu & Debian. If you have another version of Ubuntu, steps may be the same or very similar. For other Linux distros, the steps regarding apt-get will probably change.
Mac OSX is untested but it should work because all the required software has been reported to work on it. ARS itself is pure Python and OS-agnostic thus everything should work out if you comply with the requirements.
Microsoft Windows is the OS less friendly to ARS, as well as to many other open source software. Nonetheless, it is very popular so we got it to run there too! Instructions for both XP and 7 are here: Microsoft Windows.