Do the following:
1. Select the movie shape that you inserted on the slide.
2. In PowerPoint 2007, select "Animations" tab, click "Custom Animations"
button.
2.1 In PowerPoint 2003/2002, right-click the selected shape and select
"Custom Animation..." menu item.
3. In the Custom Animations pane, select the "play" animation dropdown box
and select "Effect Options..." menu item. The "play" animation for a video
looks like the right arrow head.
4. Select "From time" in the "Start playing" section and enter the time when
you want to start your video in that box.
5. Click OK button.
This will make PowerPoint start the video at the time when you want instead
of the beginning.
- Chirag
PowerShow - View multiple PowerPoint slide shows simultaneously
http://officeone.mvps.org/powershow/powershow.html