You can do this a couple ways:
1.) Login game and go to options/video setting and in here you will see options to change to window mode.

2.) Open your L2.ini file and look for [WinDrv.WindowsClient] under this your will see a few options.

You can set the size of the window made with these:

WindowedViewportX=1024
WindowedViewportY=768

Here is where you change from Full screen or Window mode:

StartupFullscreen=True -- Change this to False

Save the file and you are done, start game and should be changed to whatever you set it to.

3.) Easiest way of all just Hit Alt + Enter