Convert Text Numbers to Real Numbers
Remember that numbers in Excel are right-aligned by default, and that text is left-aligned by default. One easy way to identify those problematic text numbers in a column of what you think is composed entirely of true numbers is to select the column, go to the Home tab, launch the Alignment options by clicking the dialog launcher in the bottom right corner of the Alignment group, select Format Cells → Alignment (pre-2007, Format → Cells → Alignment), ensure that the horizontal alignment is set to Excel's default of General, and click OK. Widen the column to a reasonable width, and all true numbers will be aligned to the right while any problematic text numbers will be aligned to the left. Dates will also be aligned to the right, as a date's true underlying value is nothing more than a number.
