Working from within windows (I assume that is what you mean by "DOS Window") you can do it (I can't speak for unix/linux).
First you need to know, do you want to maximize the window but still remain in a graphics mode, or do you want to jump into a non-graphics mode? I will assume you mean a non-graphics mode since you said "full screen" and the console does not really ever go "full screen" (though you can change the height/width in characters and maximize the window and cover most of the screen).
The WIN API fucntion you need is "
SetConsoleDisplayMode", chances are that it is not defined in your wincon.h (it was not in mine anyway) but once you know the name of the API funciton you can search for it on google and find some answers:
the best version I have found is
here.
Basically you use look up the function in the Kernal32.dll and use a function pointer to access it.
There is also the option of sending a ctrl+enter key combination to the window using SendMessage(), that should also change the window to text mode.
Note that windows Vista does not like text mode applications and will probably refuse the mode change.