[Zope] zip files corrupted with IE when opened, fine when downloaded

Hi there, we have a problem with an intranet that runs using plone 2.5.3 when users using IE want to open a zip file from the intranet by using the "open" option from the download dialog they get winzip complaining that the file is corrupted. if they select "save" from the same dialog they can open the zip archive without problem. does anyone know a solution for this or give me an idea where to look for one? thanks robert