How to factory reset entire system

My xbox from 2004 is saying that there is nothing on the hard drive but it has minimal space available and if i delete everything that I have on it I still only have a few hundred blocks available instead of the 50,000 it is supposed to have. Is there a way to completely reset the system to factory settings so there is not anything left on the system or is there some place I can take it to try and get it fixed? does any one even do xbox repairs anymore?