Newbie Question

Hi,
I am planning on buying a Sherline with CNC option and convert it
to CNC. I am trying to understand how the CNC process works, this is
how I understand it works
First you draw the parts you want made in .dwg or .dxf or other
drawing format.
Next you use another application and convert the drawing to g-code
and use a g-code interpreter to convert the G-code to pulse and
direction to the parallel port.
The g-code interpreter works on DOS.
If this is right why does the g-code interpreter work in DOS?
Is there one application that them all. I am will be using the
mill/lathe for my hobby needs and I am looking for a cheap software.

Also can some one explain the options in hardware and software I have
to convert the sherline to CNC.

Thanks
Kim