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Thanks for all the replies. I found an old copy of ACDSee (v3.1) which
does exactly what I want - I can just point it at a folder on a
CD/DVD-R and it immediately shows thumbnails of the photo files in the
folder. I can then click on Tools>Slide Show and it starts showing them
one at a time. There's no need for me to set up a list of files I want
to look at, like I seem to have to in Irfanview, or copy the files to
my HDD first, etc.
Perhaps I should have been clearer in stating my requirements - I don't
want a slide show per se (I can create one on a DVD or VCD for showing
in DVD players), I just find it a convenient way to look at photos in
my archive without having to copy them to the HDD first, when I'm
trying to locate a particular one.

Anyway, problem solved and thanks for all the advice.

(not so) PI (now)