Some online streaming video can be converted to common video file formats using services such as Zamzar:
http://www.zamzar.com/url/
Embedding Video into PowerPoint
Embedding video clips into a PowerPoint presentation is quick and easy. Just remember that the length of your clip(s) will increase the total file size of your presentation.
*NOTE: Be sure to check which video formats your version of PowerPoint will accept.
Linking to Online Video
Linking to online video sources (such as YouTube) is also simple. Be aware that since the video resource is not embedded into your PPT file, it must be available online at the time your presentation is given. As online video is always subject to takedown, be sure to double check your links immediately prior to your scheduled time.
More Info
Visit Microsoft's online PowerPoint training center
Disclaimer
Words of wisdom from Mel Regnal of Colby ITS:
"Be careful about embedding video in PowerPoint presentations. Check to see what video formats your installation of PowerPoint will accept - this varies by version of PowerPoint and operating system (PC or Mac). Sometimes it is easier to link to a video file on the web rather than try to embed it into the presentation. Also, be aware that embedding a video into PowerPoint is going to increase the size of your presentation significantly."
Another friendly disclaimer:
The instructions on this page are intended for those working on educational projects and who intend to observe the practices of Fair Use of copyrighted materials. When in doubt, contact your friendly neighborhood librarian.
The Smartcopying website has produced All Right to Copy?", a resource designed to teach you about copyright, and how it impacts on you both as a user and creator.