Exec problem (using Winzip - command line)

I wrote a script to backup my Thunderbird user profile every night by zipping the whole profile (with subfolders) into a zip file. I use exec to call Winzip with command line parameters (see - http://www.memecode.com/docs/winzip.html) The script worked fine until I tried it under my Apache 2.0 installation (I usually use 1.3). The exec command starts Winzip (I can see it in task manager), but it just sits and pins - doing nothing. The Php script times out. I can not figure out what the problem could be. exec($cmd, $output, $retval) with $cmd = 'dir 'works, as does changing $cmd to another program in my Program Files folder. It appears that it is only Winzip (in Program Files/WinZip) that will not work. Any ideas what it could be?