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.
- In the insert menu, select "slides from"
- Select "other presentation"
- Here is the trick: click "select slides to insert" rather than "select all slides"
- Click insert, wait a few seconds
- 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)
- In the home ribbon, click "new slide"
- Click "reuse slides"
- Browse for the file to be inserted
- 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.