Some people have discovered that when they try to attach zip files to email, Outlook 2003 can't 'see' the files. They see every other file in the File Browser dialog except zips.
This is related to the Compressed Folders utility included in Windows and it's ability to view zips as folders. You'll need to unregister the zip handler or drag the zips from Windows Explorer to the new mail window to attach them.
To disable the feature at the Start menu, Run command, type:
regsvr32 /u zipfldr.dll
To restore the functionality, return to Start Menu, Run command and type:
See Zip Files Don’t Display in the Insert File Dialog for more information
Published May 26, 2005. Last updated on January 3, 2012.