lost my canon zoombrowser ex disc for 300d. please help

You can download it from Canon's software updates website.
http://software.canon-europe.com/products/0010037.asp

I've found with 200Gb of photos (that's just 2005) Zoombrowser is appallingly slow.
It's great for smal numbers of pictures, but not for your life's work.

I use ThumbsPlus (http://www.cerious.com/)
and FastStone (http://www.faststone.org/) for image viewing.

I've also downloaded Microsoft's image viewer to show thumbnails of RAW files.
(http://www.microsoft.com/downloads/details.aspx?familyid=D48E808E-B10D-4CE4-A141-5866FD4A3286&displaylang=en)

FastStone is free, but has an annoying bug when resizing a photo that makes it invisible to Microsoft software.

I just use Canon fileviewer utility to view photos. works well, you can tag photos you want to edit, etc.