Shaded cells changing color for no reason
 

My setup:
Dell Latitude D610 Laptop
Windows XP Professional SP 1
Excel 2003 SP1

Problem:
Spreadsheet A has some shaded cells (different shades of blue).
If I open another spreadsheet, say B, within the same instance of Excel, and
toggle back to the spreadsheet A, the cells in A have changed color all by
themselves! This is driving me nuts!

This only happens with certain spreadsheet Bs. Not all spreadsheets will
cause the cells in spreadsheet A to change color.

The two spreadsheets are completely independent, they do not link to each
other. This is driving me crazy. If I print spreadsheet A, the colors come
out as I expect. If I print the same spreadsheet after the weird color
change, the colors in the printout come out weird as well. So it's not just
a color display thing, even the data sent to the printer indicated that those
cells have changed color.

Any ideas why this is happening?

--
VA