If you export data with zip code fields and try to import that data into Excel or most any spread sheet program, you will have a problem if your zip code starts with a 0. Excel will think it is a number and simply remove the extraneous 0 from the beginning leaving you with an invalid 4 character zip code.
To fix this problem, select the column of cells with zip codes, right click and select Format. Then select a numeric format, choose a custom format and in the "Type" field enter:
00000 (5 zeros). (See this video from Microsoft for help creating a custom numeric format.).
When displaying your data, any field with less then 5 numbers will have a 0 pre-pended.