Saving video to iMac

[Zip106]

I'm about to make a DVD (in iMovie) from video I captured at a friends wedding.
This was at the time the Icelandic volcano erupted and I want to include some footage of this at the beginning of the DVD as it was touch and go wether we'd make it (wedding was in Greece).

Is there any way of saving video from Youtube or BBC website to my computer that I'd be able to use?

Thanks.