You can read about the problem here.

XL: Error Message: Too Many Different Cell Formats
http://support.microsoft.com/default.aspx?scid=213904

Nina of S-o-T wrote:

Hi, please help me, I'm on Vista Home Premium and recently Excel has started
giving me the message "Too many cell formats" when I try to merge-cells or
set font-size, etc. I've checked the formatting and can't find the problem
because each of the cells is set similarly to each other (ie font size,
typset orientation and positioning). Thanks, Nina.

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Dave Peterson