I experimented with this and discovered that when an open WoW window becomes completely covered by another window, its CPU usage goes up by more than three times. I'm doing this in XP but it matches what you say about maximizing a window. I found a work around -- when you maximize one WoW window, minimize the other one. This prevents the covered window from demanding a crazy amount of cpu time. I don't know if this applies to your situation but maybe it will help.