Check out your local settings in IE and File/Options in Explorer. This sounds
like a local thing, not portal.

I have already changed the Explorer > Folder Options > File Types >
PPT > Advanced : Uncheck browse in new window.

Let me explain in more detail -

Within document libraries the PowerPoint Presentations do open  in
Microsoft PowerPoint. The issue I am having are with Group listings
within Portal Server. They open to a full power point presentation
rather then in the Microsoft PowerPoint application.

The following is the solutions to this problem.

support.microsoft.com/

I will try to play with that sometime this week.