I regret to inform you that MS PowerPoint 2007 does not add real headers and footers, rather it adds normal rectangle shapes as headers and footers. In the current Aspose.Slides API, ShapeEx.AlternativeText property can be used to identify a shape provided the creator of the presentation adds unique "Alternative Text" to shapes. An issue with issue id 13855 has already been created on our Issue Tracking System in order to figure out any possible way to identify such headers and footers. But so far there have been no success made. This thread has been linked with this issue so that you can be updated here as soon as this issue is resolved.
Thanks and Best Regards,