Google added a new facility to view PowerPoint attachments as slideshows in GMail. So If you want to directly view the PowerPoint Presentations directly from your GMail, with a single click.
You can move through the slides with the slides show arrow buttons provided on the top, or you can directly jump to the required slide by clicking on the slide numbers listed on the right side.
Any way its cool one, you dont have to download the complete presentation.
Suggested reading: Download Free Microsoft PowerPoint Viewer 2007