Firs tof all have you run a basic SimpleViewer gallery from your c: drive to make sure its all working before trying to run it from a web server?
You can run your gallery from your c: drive by clicking on the index.html file. This will run your default browser and is called running 'locally'.
The files and folders should be like the following whether on your c: dirve or on your web server. Running it from your c: drive first lets you check that all your simpleviewer files are working. Any other problems will most likely be web server related.
your_gallery_folder
.........images
.........thumbs
.........gallery.xml
.........index.html
.........swfobject.js
.........viewer.swf
All images including thumbs should be .jpg (lower case extension) with matching file names.
Regards
Phil
http://www.phillipnorman.com