So we'll assume that you have downloaded the video in Flash format. I started to write out the steps, but then decided that the wheel had been invented. So here are instructions from http://support.microsoft.com/kb/291875.
PowerPoint 2003, 2002, and 2000
To add a Shockwave Flash Object control to a slide, follow these steps:
1. Start PowerPoint, and then locate the slide that you want to insert the control into.
2. If the Control Toolbox is not already visible, point to Toolbars on the View menu, and then click Control Toolbox.
3. In the Control Toolbox, click More Controls (which looks like a hammer and wrench), and then click Shockwave Flash Object.
Note: Shockwave Flash must be installed on your computer for the Shockwave Flash Object to be listed in the Control Toolbox.
4. Draw the control on your slide. The Shockwave Flash Object ActiveX control now appears on your slide.
To make the Shockwave Flash control play back your Flash animation file, follow these steps:
5. Right-click the inserted Shockwave Flash control, and then click Properties.
6. Click the Movie property. In the Value box, type the full drive path, including the file name (for example, C\:MyFile.swf) to the Flash file that you want to play.
7. Make sure that the Playing property is set to True.
Make sure that the Flash Player is installed on the computer. Then, follow these steps:
1. In PowerPoint, display in normal view the slide on which you want to play the animation.
2. Click the Microsoft Office Button at the top left, and then click PowerPoint Options.
3. Click Popular, and then click to select the Show Developer tab in the Ribbon check box under Top options for working with PowerPoint, and then click OK.
4. On the Developer tab, click More Controls (which looks like a hammer and wrench) in the Controls group.
5. In the list of controls, click Shockwave Flash Object, click OK, and then drag on the slide to draw the control.
6. Resize the control by dragging the sizing handles.
7. Right-click the Shockwave Flash Object, and then click Properties.
8. On the Alphabetic tab, click the Movie property.
9. In the value column (the blank cell next to Movie), type the full drive path, including the file name (for example, C\:MyFile.swf) to the Flash file that you want to play.