This behavior usually starts after you run a DOS-based program (in DOS-only "compatibility" mode) which, for some reason, terminates abnormally. Sometimes, the computer will automatically launch the DOS program upon bootup.
1) Type "Exit" at the command prompt,
2) If this does not solve the problem, edit the file "Config.sys":
"c:\edit config.sys" and remove the line "DOS=SINGLE", save the file, exit and type "Exit".