to many different cell formats

I have a question concerning an error message plaguing me: "too many different cell formats".
I have version Excel 2002 running on XP. I use excel in a fairly basic way, basic formulas, multiple sheets, colour coding cells, font differences between cells, manipulating borders. All cells are set to "general" format. Intermittantly and becoming more regular when I go to (for example) put a border around a cell or colour highlight the cell I get error message "too many different cell formats" and no changes are accepted. Any advice?