You probably noticed even under safe mode with command prompt you kind of have a GUI (the command line is in a window). There's not really any way to do it differently as this is Windows 2000/XP-based. (See this
The only place you'll see a "pure" command line like in DOS would be recovery console, but you've probably noticed it's very restricted and is only allowed to run a limited number of commands.
In any case, to do what you want, you can log into the account in which you want to use the command prompt only, then run the registry editor (start run, regedit). Navigate to this key:
Create a new string (edit, new, string value). Name it shell, and for value put cmd.exe
Logoff and back on to see the changes.