You already know that Turbo c is junk and running it via DOSBOX will only make it 'junker' since some programs produce runtime errors.I use visual c++ 2008 express edition for practicing most of the coding and if a situation comes where using turbo c is unavoidable,only then do i use it and in the windowed mode.Still if you want to dosbox it here are the steps.
(Better if you install TC++ at d:\tc follow the steps)
1.Copy paste the contents of c:\tc\tc into c:\tc itself
2..Make a folder named turboc in C:\ and cut and paste the folderc:\tc into this turboc folder
3.right click on the desktop select new-->shortcut
i)type the location of the item as "x:\Program Files\DOSBox-0.74\DOSBox.exe" -c "mount c c:\turboc" -c c: -c "tc\tc" [replace 'x:\Program Files\DOSBox-0.74\DOSBox.exe' by the actual installation path of dosbox]
ii)click next, enter a name for the shortcut and click finish
iii)Open this newly created shortcut whenever you want to run turbo c