To DIY your windowed mode,
1. Download L2decrypt from
http://web2.249088.vserver.de/?f=l2decrypt
2. Drag your L2.ini in your L2 system directory onto l2decrypt.exe
3. Press enter to close l2decrypt
4. Look for l2.clear.ini in your L2 system directory and open it using a text editor.
5. Look for this line "StartupFullscreen=True" and change the "True" to "False". There's 2 of this line i think.
6. Save the file.
7. Drag it back onto l2decrypt.exe and press enter
8. Backup your L2.ini
9. Rename l2.crypt.ini as l2.ini
10. Your L2 is now in windowed mode.