Thanks for the info and link, it might be helpful for some scenarios.
However, as it states that's for Windows Vista or Windows 7 using Windows Mail or Windows Live Mail, and apparently because the installation of the PowerPoint Viewer did not create a necessary registry key, etc.
The problem that Paulinea was having usually occurs with the Yahoo! or HotMail type web based e-mail services where the .pps or .ppt file can not access a required .dll file on a users computer when it's opened in their temporary cache. I assume that they do not want to allow any type of executable file within their temporary cache for valid (and understandable) security reasons.