If you really need a basic image viewer, with only a few bells and whistles, try Slowview from
http://slowview.at
Its nowhere as advanced as ACDsee or irfanview, but it does its job very well. It shouldn't be to hard to set up the slideshow, because the options consist of a slider to set the amount of time between images, which direction you want it to go (sequentially up, down, or random), and whether to start over again when it finishes a directory.
The worst part of the program, though, is when you have a large directory of images. If you have preview mode set on (start program and hit Enter), then it will load and cache all the images in the open directory. If you don't have it on preview mode, then it will load each image as it come in the slideshow.
You can do other stuff with the program, like batch image conversions (bmp to gif, gif to jpg, etc), but its main purpose is to just view images outside of your internet browser, and to perform slideshow duties.
edit: oh yeah, and its FREE FREE FREE!!!