1. Select the column header (A,B,C,...), for the column that has the leading zeros, to highlight the entire column
2. Right-Click on the column header
3. Select 'Format Cells'
4. Select the 'Number tab'
5. In the 'Category' list, click 'Custom'
6. In the 'Type' box, replace the word 'General' with the number format, such 00 for a two-digit code, or 0000000 for a seven-digit code.
Warning: Do not re-open the file or the zeros will get removed again.
Video - How to Work with Leading Zeros