Here's some additional information about this problem:

1. It's happening to PowerPoint 2010 too.

2. So far it has been happening only to files that were created on other PCs (many different PCs). It does not happen with files created on any of my PCs.

3. If I open one of these files in Word 2007 and then Save it I am then able to open it in 2010.

Again, no problem opening any of the files with Word 2007. The problem is only with 2010.

Any clues appreciated!

Peter