If you're in plain DOS, it's likely that you don't have a DPMI server
  installed, which is required to run TETRIS.EXE.

  That is why I have included CWSDPMI, which will be loaded automatically if
  needed.  CWSDPMI is free; the full binary distribution and the source code
  can be downloaded at http://clio.rice.edu/cwsdpmi/.

  See CWSDPMI.DOC for more info.

  TETRIS16.EXE is a 16-bit version which uses the ANSI.SYS driver.  You will
  just get garbled text if it is not loaded.  There should be a line like
  this in your CONFIG.SYS:

        DEVICE=C:\DOS\ANSI.SYS

  There are also free replacements available which might work even better,
  NANSI.SYS and NNANSI.SYS.