To the guest with whom I was just chatting: my apologies; I hit a wrong button and lost our chat. See http://factfinder.census.gov/jsp/saff/SAFFInfo.jsp?_submenuId=aboutdata_3&_pageId=geography for an explanation of what falls within counties. Most likely you'd want a file linking counties and county subdivisions. I don't recommend using zip codes as they don't directly relate to counties, as the graphic shows. Please do try to contact me again for more assistance (though I suppose email might be better this time) and I apologize for cutting off our conversation!