You can try downloading the videos from Youtube and playing the videos in your regular player. There's dozens of services and add-ons that do that (none of which should cost anything). You can even convert the files to a format that Powerpoint recognizes (.asf, .avi, .wmv, or .mpeg) and play it directly from the slide. If you're on a school computer and don't want to install anything, get a portable version from portableapps.com which you can run from a USB stick or put in a temporary directory.