How can I put a PowerPoint presentation with a recording of the presenter?
Any timeline-based multi-track video editor with crop and scale features will allow you to do this. Basic idea is to import all three assets viz. video, logo and Powerpoint (see below) on different tracks in the editor and resize/crop each of them so that the composition looks like the DEFCON videos.

The time-consuming part is syncing the slides with the presentation. That may be simple but not quick.

For Powerpoint, one has to export individual images of all slides, if there's no animation or transition in the PPT that you want to preserve, or export a video if you do. For video export, you need 2007 or later version of Powerpoint, I believe.

All professional video editors (Premiere, FCP, Avid, Vegas, Edius..etc) can do this. Here's a list. I'm not aware which of the free or lite video editors are best suited for your task, sorry.