First: Download and install Winrar if you haven't already.
Second: Download this Winrar file I've provided.
Third: Open up the file called dxwnd.exe
Fourth: Click edit, add, click the DirectX8 option (corresponding to your version of DirectX I think, but I think majority of people have directX8 )
Fifth: Click the ... button at the top right corner, find the maplebreak client, click Open, and then press the Ok button just below the ... button.
Sixth: Keep DXWnd opened and open the maplestory client. Now it should be windowed! Whenever you want it to be full screen just don't open DXWnd and whenever you do keep it open.
Guarantee lagless (although I think some servers have protection against DXWnd and takes it as a possible hack I'm not sure, but I know it works perfectly fine on Maple Break.)
Hope you guys like it,