This worked for me (Win7 x64, dual widescreen monitors, Radeon HD 6970): - Fresh 0.20 Steam install with no addons (those come next, muhaha!) - I ran the game and set graphics to 1920x1080 with fullscreen disabled, hit apply/accept, and restarted it. KSP still started fullscreen. - I closed the game, then added -popupwindow to my desktop shortcut and started the game again (run as administrator is checked in the shortcut's compatibility tab.) - For posterity, my shortcut's target field reads "D:\apps\steam\steamapps\common\Kerbal Space Program\KSP.exe" -popupwindow - In-game, I went into settings and set the resolution to 1024x768, then hit apply and accept. Then I set it back to 1920x1080 and hit apply and accept. - Now KSP is running at 1920x1080 in a borderless window. Since then, I've restarted the game (still using the popup window shortcut option) and it kept the setting. I hope this helps.