If you are entering values that start with a leading zero, such as United States zip codes or Social Security Numbers, you need a way to display the full number including the leading zero, and stop Excel from automatically hiding the leading zero when you enter such a number.

To have your entries be the way you enter them without intervention from Excel, either enter a preceding apostrophe ' character followed by the actual number (example '01234) or format the cells as Text before doing the data entry.

To format as cells as Text, select the cells.

For Excel versions 2003 or before, from the worksheet menu, click Format > Cells > Number tab.   Select Text from the Category pane and click OK.

For Excel version 2007, from the Ribbon click the Home tab, and in the Cells section, click Format > Format Cells > Number tab.   Select Text from the Category pane and click OK.

You can also apply a custom format to the range by following the above steps, but instead of Text, select Custom and enter "00000" (without the quotes) if, for example, you are entering five-digit zip codes.   Note that formatting only affects how you see what you entered, not what you actually entered.   Example, with this custom format, entering 02567 or just 2567 will appear as 02567 in the underlying cell value will really be 2567.