Merging different PowerPoint presentations usually creates format chaos. The slides that are inserted are automatically re-formated in the style of the mother presentation. Especially if you want to make a collage of different presentations with a completely different look and feel, this is a disaster. Simply "ctrl-c / ctrl-v" slides across does not work. One solution is to PDF both presentations, then merge them in Adobe Acrobat. But there is a solution inside PowerPoint as well.
  1. In the insert menu, select "slides from"
  2. Select "other presentation"
  3. Here is the trick: click "select slides to insert" rather than "select all slides"
  4. Click insert, wait a few seconds
  5. A window will open from which you can select slides, at the bottom you should click the box "keep design of original slides"
PC (PowerPoint 2007)
  1. In the home ribbon, click "new slide"
  2. Click "reuse slides"
  3. Browse for the file to be inserted
  4. Click the "keep source formating" box
UPDATE: PowerPoint Join seems to be an automated tool developed by Sobolsoft that links PowerPoint presentations together. I have not tried it though.