You mean windowed mode so you can do other things while running it in background?
When you open the game, in the screen before you click Rank/Local, there's like 4 options, normally you just hit the top left one. Click on "configurations" on the right side instead. Under videos, you need to "uncheck" Full screen mode and check the resolution/size you want the game to run in. That's it.
edit: wait, it might be "options" instead of "configurations." Configurations appears right after you first open DD.exe. You might be able to do it from that screen as well, but going through the options is easy enough.
This post was edited by downding on Feb 1 2012 04:32pm