Make sure your game is closed before doing that.
Open the config file with the notepad to edit it.
plutonium.cfgis for singleplayer/zombies and
plutonium_mp.cfgis for multiplayer
Change these 4 dvars to be like below (you can use ctrl+f to search for them)
seta r_fullscreen "0" seta r_noborder "1" seta vid_xpos "0" seta vid_ypos "0"