Slide show on web site

I created a PowerPoint slide show of photos in PowerPoint 2007 and loaded it
to a web site. When I went to view it, it opened as a WinZip file. I
recreated the slide show using PowerPoint 2003 (I have both on my pc) and
when I went to view the 2003 version from the web site, it opens in
PowerPoint itself, rather than in the viewer. I noticed that the File
Download window asks if I want to open a '.ppt' file rather than a '.pps'
file and therefore assume that this is why PowerPoint opens rather than the
viewer.
I have set my pc to always open '.pps' and '.ppsx' in PowerPoint Viewer.
When I loaded a slide show to the web site last year, I did not have
PowerPoint 2007 on my pc; this leads me to think that there is some setting
in the 2007 version that is causing the problem.
Can anyone suggest what I can do to correct this?
Note that the slide show opens successfully on pcs that do not have
PowerPoint installed, only the Viewer.
--
Elaine