I'm running Excel 2007 with Windows XP Pro on AMD 64 x2 dual core 4800, 2 GB ram. Occasionally I get the "Not responding" message. I'm probably pushing performance with my sheets but I would like to know where the bottleneck may be. Please refer to the attached Word.doc for a shot of the Task Manager at the time of "Not responding" (scroll up the page). The only aspect which is recognisable is cpu usage - the rest is meaningless. Maybe the clue lies in these unknowns. The Task manager shows cpu usage at around 50%. Is near 100% not possible? Excel is set to run dual core. Grant.