This problem occurs on a Dell Precision M70 laptop running Windows XP SP2 & Excel 2002 (Office XP). The video card in this unit is an nVidia model, and the drivers have been updated to the latest rev as posted by Dell.
When docked or in standalone mode, performance is normal when using Excel or any other application. However, when this laptop is connected to an Epson projector, the screen automatically zooms to 400% normal view when a cell is selected or the worksheet is scrolled. No amount of toggling between the laptop display and the projector will reset the video back to the default. The overall Windows desktop appears to take on a low-resolution, large size icon scheme when this problem happens in Excel.
The only 'fix' I've found thus far is to shut down cold, which will restore the laptop to it's normal display mode.
There is a similar posting in one of the forums for a PowerPoint problem, but I require more than a 'adjust the video settings' suggestion for a solution.
Thanks, and good luck.