I generally write any presentations I do in Keynote.
I'm not a fan of Microsoft Office at the best of times - it's awful in Windows and not a lot better on the Mac (my chosen desktop platform).
Mostly I'll send presentations to clients as a PDF, but sometimes I need to export as a PPT. This used to work fine, but today I discovered that every export produced a useless PPT. Powerpoint, on both Windows and Mac, would complain the file was corrupt and couldn't display any graphics or text.
A trawl through the Apple discussions threw up this conversation - and it turns out the root of my problem. Embedded text and graphics. Everywhere I had over-layed a text box and another text box or a graphic would produce this problem. Making sure nothing was overlayed and the export problem was solved.