Not sure that this will work, but maybe worth a try:
Office 2010 PowerPoint. Some 1997-2003 ppt files will not open. - Microsoft Community
I found this to be a patch to the problem...
Easier solution to the above problem. It would appear that when downloaded normally, the browsers are flagging the file as being potentially unsafe, whereas DownThemAll! is not. It would also appear the the PowerPoint 2010 programmers failed to handle this in the code. So, the easiest way to get around this error is to navigate to the file on your computer, right-click it and go down to properties. In the box that pops up, go down to the security section (at the bottom) and unblock the file. Open file.