If you have a data file "ziplnglat" that lists all the zips with their long and lat (purchased) and you know the lng and lat of a particular zip code and a radias, here is some sort-of code that retrieves the other zip codes with in the specified radias.
The ziplnglat table has three columns zipnumber, lat and lng

mtlat=request.form("slat")
mtlng=request.form("slng")
mrad=request.form("srad")
SELECT Ziplnglat.zipnumber 
FROM ziplnglat
WHERE 
((69.1*(lat-"&mtlat&"))**2+(69.1*(lng-"&mtlng&")*COS(DTOR("&mtlat&")))**2)**0.5 < "&mrad&")