That is what I thought you might prefer but thought I'd check. With the custom slide show you need to go into the Custom Slide Show dialog and activate the show you want to watch so the audience would see that.

If you are not sure how many times you want the first few slides to loop then to make it as simple as possible and so your audience see's a smooth presentation, I would put the slides you want to loop into a separate PSTX file and set that to loop through Set Up Slide Show and then have the second set of slides in another PSTX file and have that already in Slide Show mode. Run your loop presentation in slide show mode and then when you want to stop that and begin your presentation simply go to the keyboard and press Alt + Tab and move to your main slide presentation and begin from there. It will leave your slides looping still in the background but it won't be visible to your audience.