Excel 2007 Workbook with 19 linked sheet that derive a Long Term Finanacial Plan - When cell selected, instead of one square being highlighted several squares are. Occurs when using a mouse. Fixed if user switches to keyboard.
Not much to go on. However, I would guess that you are clicking on a merged cell.
Select the cell in question and check out the "Merge & Centre" button in the "Home" bar. If it is highlighted, it indicates a merged cell.
The other alternative is that there is an overlayed object - in that case, when you click on it, it generally will show selection marks around it.