Microsoft Power Point Viewer 2003 or 2007.. Problems...

I don't have MS Office installed so I installed first MS PP Viewer 2003 and then 2007 with the same results.. When I receive an email with a PPS attachment in Outlook Express and try to open it, I get a message telling me to go to Control Panel and associate a program with the extension type to view it..  I have done this many times and for the PPS extension, MS Power Point Viewer 2007 is assigned to it..BUT... Outlook Express still won't open the attachment.. I have 3 other computers wher I have done this with no problem, but this time it won't work...  If I download the attachment to the desktop, then it will open... Any idea what is going on?