Outlook 2003 pptx attachment won’t open

Hello,
I have Outlook 2003 and was emailed a pptx attachment. When I double click it, it always asks to save instead of opening it. Docx files open fine. I have the Office 2007 compatability pack installed. If I save the pptx file to my desktop i can open it from there, but not if I double click the attachment. Thanks for your help.