If you are running NT, 2000, XP or Vista, there is no DOS mode, unless you had a previous installation of DOS, 3.11, 95, 98 or <gulp> ME and set it up as dual boot.
You should be able to boot off a windows CD to reformat and reinstall.
adjust the startup BAT report utilising a text fabric editor (like Notepad) locate the line that launches domicile windows and positioned an apostrophe ( ' ) in front of that line... the computing device will forget approximately with reference to the line. shop the diversities and reboot.