|Keep tapping F8 as soon as you turn on your PC, until the Advanced Startup Options appears. |
Using the up/down keys, highlight "Safe Mode with Command Prompt"
Then press <Enter> key.
This opens a command prompt window without loading the normal Windows interface.
However, as others have said, to use "pure" DOS on a Windows 7 PC you would need to use a bootable CD containing a version of DOS.