Can I have The nxt compiler AND the arduino compiler on the same machine?

You mean ROBOTC for NXT and ROBOTC for Arduino, installed on the same PC? Yes! From what I understand, ROBOTC 3.x is all the same package regardless of the target platform, you just enable different parts when you get different licenses.

mattallen37 is correct; when you install a version of ROBOTC, you are installing the core program and all of the supported platforms (NXT/TETRIX, PIC/Cortex, Arduino, etc). Then, if you would like to use a different compiler, all you need to do is activate it through ROBOTC's Add License option (Help-> Manage Licenses -> Add License). This way you can easily add new functionality to ROBOTC without having to go through multiple installs; once activated, all you have to do is change your Platform Type inside of ROBOTC and start coding for the selected platform!

We actually have a walkthrough guide on this you may want to check out, it's located here: http://www.robotc.net/wiki/Tutorials/Ge ... ate_Online

Please, do not hesitate to let us know if you have any questions or run into any issues at all.