You can do this with Custom Show.
Lets say the first half of the slides will be automatic slide show, while
the second half will be the actual presentation.

First, click Slide Show > Set Up Show.
Put a checkmark to Loop continuously until 'Esc'.
Click Ok.
Now, Click Slide Show > Custom Shows.
Click New.
Select first half of the slides, click Add.
Click Ok and Close.
Hold down Shift key, select the first half of the slides.
Right click on the slides, select Hide Slides. (This will prevent them from
showing up unless you are in the custom show)
Click Slide Show > Slide Transition.
Under Advance slide, put a checkmark to Automatically After. This will allow
the slides to loop automatically.
Do NOT Apply to all slides.
You will need to insert a new slide just before the slides you are looping.
On the new slide, insert an action button.
Set the hyperlink to the custom show you have created.
Put a checkmark on "Show and Return".
Ok your way out.

Now, view the slide show.
If you click on the action button, you will go into the custom show, which
will run automatically. Press the Esc key to exit the custom show, and mouse
click or enter key to continue with your actual presentation.

Alan,

see tutorial on how to loop a set of slides (while people assemble in the
room), then jump to your real presentation
http://www.awesomebackgrounds.com/powerpointlooping.htm

Cheers
TAJ Simmons
microsoft powerpoint mvp

awesome - powerpoint backgrounds,
http://www.awesomebackgrounds.com
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