Run game in a window
To run the game in a window on your desktop, enter "-w" at the command line.
So, if you have a shortcut on the deskop you will want to put the code in this way:
"C:\Program Files\Diablo2\Diablo2.exe" -w
NOTE: The directory where the game is installed will vary per computer. Also, be sure to put the -w in after the quotes.