Windowed mode for L2 is easy as pie.
Click your Windows key (or Esc + Ctrl if you have an older keyboard) to return to the desktop. Rightclick on the Lineage II icon on the start-bar and leftclick on Maximize. L2 will go into Windowed mode. This works on Win XP just fine.
To go back into fullscreen, just rightcllick on the Lineage 2 icon again and click maximize again – and shazaam – back to fullscreen.
100% legal too!