Problem with worksheet names containing charts
 

In some of my workbooks, I have multiple charts, each in a different
worksheet. In Excel 2003 I had each of these sheets named. Excel 2007 has
lost these names and is calling the worksheets "chart 1", "chart 2" etc etc.
Each attempt at renaming has worked initially, but after a restart of my
computer the names are lost and worksheets are again named "chart n", "chart
n+1" etc etc.