SCSI Zips can be quite a problem; I've been fighting with mine for a while, now.  I'd suggest that you remove all the software associated with it (you might keep the drivers at first) and disable "auto insert notification" for that drive, which may prevent it from trying to usurp the functions of your CD drive.  You may also need to fiddle with the BIOS to stop the Zip (they are aggressive things, largely because Iomega hasn't bothered to update them to take into account the fact that other manufacturers use SCSI, too, and a more recent version) as well) from believing that, not only is it the only removable disk drive on your system, but really ought to be your hard drive as well.  Try those suggestions in order (but don't touch the BIOS until you've done a lot of research), and good luck!

Malcolm