How about disabling left click on the last slide of the custom show?
To do so, go to the last slide, click Slide Show > Slide Transition.
Under advance slide, uncheck on mouse click.

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Shawn Toh (tohlz)
Microsoft MVP PowerPoint
Site Updated: Feb 20, 2008
(Amazing PowerPoint animations, artworks, games here)
http://pptheaven.mvps.org
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> Hi,
> I am using PPT XP. I have several custom shows in one main presentation (has
[quoted text clipped - 13 lines]
>
> Martha
Unchecking the option will disable only mouse click, not scrollwheels. My
apologies. However, if you do not want the custom show to end unless a button
is clicked, you will need to either loop it, or set it to kiosk mode (Slide
show > Set Up Show).

Signature
Shawn Toh (tohlz)
Microsoft MVP PowerPoint
Site Updated: Feb 20, 2008
(Amazing PowerPoint animations, artworks, games here)
http://pptheaven.mvps.org
PowerPoint Heaven - The Power to Animate
> Hi,
> I am using PPT XP. I have several custom shows in one main presentation (has
[quoted text clipped - 13 lines]
>
> Martha
MarthaH - 24 Mar 2008 19:42 GMT
Looping it only seems to work if I have only 1 custom show - I have 8. I
thought of Kiosk mode, but am unable to manually advance the slides using it.

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Martha
> Unchecking the option will disable only mouse click, not scrollwheels. My
> apologies. However, if you do not want the custom show to end unless a button
[quoted text clipped - 18 lines]
> >
> > Martha