If what you need is to run a simple c/c++ software, you have to know that the arduino environment support c/c++ code, as explained in the arduino homepage http://code.google.com/p/arduino/ If for your requirements, you need to use a real time operating system, there is a contikiOS version for Arduino (Contiki-Arduino).
Last but not least, if you are a beginner, i would advice you to use a popular RTOS, to find support on internet easily if you need it. Sadly, i am not an expert on that field. But when i google it, i find Duinos, maybe you could have a look on it.