If you have a YouTube video which you want to play it during the presentation, you can make use of the Shockwave Flash Object ActiveX Control.
To do so, open up PowerPoint.
Click View > Toolbars > Control Toolbox.
Click on the last icon, 'More Controls'.
Scroll down and look for Shockwave Flash Object.
Select the object, then insert it onto the slide.
Once you are done, right click on the object and select Properties.
Open Internet Explorer and look for the YouTube link. Copy the URL which should be something like: http://youtube.com/watch?v=0_jjLOqm2L4
Go into PowerPoint and double click on Custom in the Object's Properties.
Paste the URL under Movie URL.
Amend the url by replacing 'watch?v=' to 'v/' : http://youtube.com/v/0_jjLOqm2L4
Ok your way out and close the properties dialog box.
Now, you will be able to view the YouTube video in slide show!