In Windows Explorer, right click on any of your 2003 PowerPoint presentations, click on properties, and where is says open with, navigate to your PowerPoint 2003 executable program location and select it as the program to open this type of file.

 I have both 2007 and 2003 on my machine, but I want the default program for
 all .ppt files to be 2003.

 I've tried going to properties and browsing to select PPT 2003 in the
 program files to be the default, but it still pops up in the properties
 panel as the icon for 2007. And things still open with 2007 even if on a
 case by case basis, I do "open with..."

 The only solution seems to be to drag an individual file onto the 2003
 program icon...

 Is there a way to force it to always open with 2003...?

 - Nolan

This is the answer that was given a few weeks ago. I haven't tried it
yet so I can't say if it works:

Go to start Run and type with quotes

"C:\Program Files\Microsoft Office\OFFICE11\POWERpnt.exe /regserve"

Click ok

This re-registers it as the default for opening PPT files.  I did the
same
for word and excel.  Woks great.