DOSBox is PC emulator with builtin DOS for running old DOS games or programs.
The installation is very simple:
# pacman -S dosbox
First put your favourite old game/program in a dir, then run dosbox with parameter saying to mount the dir:
$ dosbox ./theoldgame/
That's it. You're in the DOS commandline and you can start playing your favourite oldies.
For more information visit the official DOSBox site: http://dosbox.sourceforge.net/