I do quite a lot of customer presentations. I have also watched hundreds over the years. I don't use MS Powerpoint these days, I use the open standards based presentation tool that comes as part of Lotus Notes 8, but like Hoover and Biro, I guess Powerpoint has become a generic term these days to mean computer based presentation graphics tool.
Geek, engineer and ex chip designer Don McMillan has some pretty funny observations on how not to construct slides and present. In fact in 4 hilarious minutes he gives more and better advice than I have seen in many formal presentation skills courses.
With his wit and insight I can see why he dumped designing processors and swapped it for performing corporate comedy routines. Here is some more of his stuff.