A 20 minute presentation is usually to short to carry a live product demo. You might run into technical difficulties, and you are losing time/attention with banal product features such as logging in, etc.

In those cases it is better to use application screen shots, rather than the real thing. You can still point at your computer and say that there is a working prototype, and that you are more than willing to take people through in a separate, longer meeting.

The next level up is to crop/magnify the screen shots and focus only on those aspects you want the audience to see. Your Skype window, menu bars, and all other unnecessary screen real estate can be cut out.

Next level up: put big, bold, explanation arrows explaining what the user should see. Say that it is a really minimal UI, if it is minimal.

Now here is the big difference. Do not just structure your product demo alongside the feature list of your app. Instead, create a user story and let the app screens flow with the story. You can also include visual images that are not screen shots into this story board. Even better, link back to story elements you have used elsewhere in the presentation.

A product demo is a user story, not a list of features.